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Published: December 2, 2022, Last updated: October 29, 2026
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[Source](https://developer.mozilla.org/docs/Web/HTML/Reference/Elements/script/type/speculationrules)
The Chrome team has brought back full prerendering of future pages that a user is likely to navigate to.
> [!NOTE]
> Prefer video to reading? Check out this [Google I/O video](https://youtu.be/bSua9vMdrNM) for a summary of what's on this page.
> [Video](https://www.youtube.com/watch?v=bSua9vMdrNM)
## A brief history of prerender
In the past, Chrome supported the `` resource hint, however it was not broadly supported beyond Chrome, and it wasn't a very expressive API.
This legacy prerendering using the link `rel=prerender` hint was deprecated in favor of [NoState Prefetch](https://developer.chrome.com/blog/nostate-prefetch.md.txt), which instead fetched the resources needed by the future page, but did not fully prerender the page nor execute JavaScript. NoState Prefetch does help improve page performance by improving the resource loading, but won't deliver an *instant* page load like a full prerender would.
The Chrome team has now reintroduced full prerendering back into Chrome. To avoid complications with existing usage, and to allow for future expansion of prerendering, this new prerender mechanism won't use the `` syntax, which remains in place for NoState Prefetch, with a view of retiring this at some point in the future.
## How is a page prerendered?
A page can be prerendered in one of four ways, all of which aim to make navigations quicker:
1. When you type a URL into the Chrome address bar (also known as "the omnibox"), Chrome may automatically prerender the page for you, if it has high confidence you will visit that page, based on your previous browsing history.
2. When you use the bookmarks bar, Chrome may automatically prerender the page for you on holding the pointer over one of the bookmark buttons.
3. When you type a search term into the Chrome address bar, Chrome may automatically prerender the search results page, when instructed to do so by the search engine.
4. Sites can use the [Speculation Rules API](https://github.com/jeremyroman/alternate-loading-modes/blob/main/triggers.md#speculation-rules), to programmatically tell Chrome which pages to prerender. This replaces what `` used to do and allows sites to proactively prerender a page based on speculation rules on the page. These can statically exist on the pages, or be dynamically injected by JavaScript as the page owner sees fit.
In each of these cases, a prerender behaves as if the page has been opened in an invisible background tab, and then is "activated" by replacing the foreground tab with that prerendered page. If a page is activated before it has fully prerendered, then its current state is "foregrounded" and continues to load, which means you can still get a good head start.
As the prerendered page is opened in a hidden state, [a number of APIs that cause intrusive behaviors](https://wicg.github.io/nav-speculation/prerendering.html#intrusive-behaviors) (for example, prompts) are not activated in this state, and are instead delayed until the page is activated. In the small number of cases where this is not yet possible, the prerender is canceled. The Chrome team is working on exposing prerender cancellation reasons as an API, and also enhancing DevTools capabilities to make identifying such edge cases easier.
> [!NOTE]
> **Note:** Prerendered pages should be treated as a hint and a progressive enhancement. Unlike `preload`, prerender is a speculative *hint* and the browser may choose not to act upon the hint based on user settings, current memory usage, or other heuristics.
### Impact of prerendering
Prerendering allows a near-instant page load as shown in the following video:
Example impact of prerendering.
The example site is already a fast site, but even with this you can see how prerendering improves the user experience. This can therefore also have a direct impact on a site's [Core Web Vitals](https://web.dev/articles/vitals), with near zero LCP, reduced CLS (since any load CLS happens before the initial view), and improved INP (since the load should be completed before the user interacts).
Even when a page activates before it is fully loaded, having a head start to the page load, should improve the loading experience. When a link is activated while prerendering is still happening, the prerendering page will move to the main frame and continue loading.
However, prerendering does use additional memory and network bandwidth. Be careful not to over-prerender, at a cost of user resources. Only prerender when there is a high likelihood of the page being navigated to.
See the [Measuring performance](https://developer.chrome.com/docs/web-platform/prerender-pages.md.txt#measure-performance) section for more information on how to measure the actual performance impact in your analytics.
### View Chrome's address bar predictions
For the first use case, you can view Chrome's predictions for URLs in the `chrome://predictors` page:
 Chrome Predictors page.
Green lines indicate enough confidence to trigger prerendering. In this example typing "s" gives a reasonable confidence (amber), but once you type "sh" then Chrome has enough confidence that you nearly always navigate to `https://sheets.google.com`.
This screenshot was taken in a relatively fresh Chrome install, and filtering out zero confidence predictions, but if you view your own predictors you will likely see considerably more entries, and potentially more characters required to reach a high enough confidence level.
These predictors are also what drive the address bar suggested options you may have noticed:
 Address bar 'Typeahead' feature.
Chrome will continually update its predictors based on your typing and selections.
- For a greater than 30% confidence level (shown in amber), Chrome proactively preconnects to the domain, but does not prerender the page.
- For a greater than 50% confidence level (shown in green), Chrome will prerender the URL.
## The Speculation Rules API
For the Speculation Rules API prerender option, web developers can insert JSON instructions onto their pages to inform the browser about which URLs to prerender.
### URL lists
Speculation rules can be based on lists of URLs:
```html
```
### Document rules
Speculation rules can also be "document rules" using the `where` syntax. This speculates links found in the document based on `href` selectors (based on the [URL Pattern API](https://developer.mozilla.org/docs/Web/API/URL_Pattern_API)) or CSS selectors:
```html
```
> [!NOTE]
> **Note:** The Speculation Rules API used to include a `source` key which was set to `list` for list URLs or `document` for document URLs but since this can be implied from the presence of a `urls` or `where` key (which are mutually exclusive), [this requirement was removed from Chrome 121](https://developer.chrome.com/blog/speculation-rules-improvements.md.txt#optional_source). You may still see this `source` key used on some sites or example code.
### Eagerness
Browser Support
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[Source](https://developer.mozilla.org/docs/Web/HTML/Reference/Elements/script/type/speculationrules#eagerness)
An `eagerness` setting is used to indicate when the speculations should fire, which is particularly useful for document rules:
- **`conservative`:** This speculates on pointer or touch down.
- **`moderate`:** On desktop, this performs speculations if you hold the pointer over a link for 200 milliseconds (or on the [`pointerdown`](https://developer.mozilla.org/docs/Web/API/Element/pointerdown_event) event if that is sooner, and on mobile where there is no `hover` event). On mobile, we changed to base this on [complex viewport heurisitics](https://crbug.com/372053392) from August 2025. Complex viewport heuristics trigger 500ms after the user stopped scrolling, for anchors within 30% of the vertical distance from previous pointer down, where anchors are at least 0.5x as big as the largest anchor in the viewport. As [described in this document](https://docs.google.com/document/d/1YPbtUPfZIDElzBZNx8IQMzRFvy8oauLG_i1XIr6jgTs/edit?tab=t.0).
- **`eager`:** This eagerness used to behave identically to `immediate` but changed from Chrome 143. On desktop, this performs speculations if you hold the pointer over a link for 10 millisecond. On mobile, we changed to base this on [simple viewport heurisitics](https://crbug.com/436705485) from January 2026. Simple viewport heuristics trigger 50ms after the anchor enters the viewport.
- **`immediate`:** This is used to speculate as soon as possible, that is, as soon as the speculation rules are observed.
The default `eagerness` for `list` rules is `immediate`. The `eager`, `moderate` and `conservative` options can be used to limit `list` rules to URLs that a user interacts with to a specific list. Though in many cases, `document` rules with an appropriate `where` condition may be more appropriate.
The default `eagerness` for `document` rules is `conservative`. Given a document can consist of many URLs, use of `immediate` for `document` rules should be used with caution (see also the [Chrome limits](https://developer.chrome.com/docs/web-platform/prerender-pages.md.txt#chrome-limits) section next).
Which `eagerness` setting to use depends on your site. For a lightweight, static site, speculating more eagerly may have little cost and be beneficial for users. Sites with more complex architectures and heavier page payloads may prefer to reduce waste by speculating less often until you get more positive signal of intent from users to limit waste.
> [!CAUTION]
> **Caution:** Over-speculation has a clear cost to users for bandwidth, memory, and CPU costs, but also for sites themselves. Be conscious of the impact you are putting on your users when implementing speculations. The `eagerness` settings allow you to reduce the likelihood of wasted speculations. Chrome also places [limits](https://developer.chrome.com/docs/web-platform/prerender-pages.md.txt#chrome-limits) on speculations to reduce the impact of overuse.
The `moderate` option is a middle ground, and many sites could benefit from the following speculation rule that would prerender a link when holding the pointer over the link for 200 milliseconds or on the pointerdown event as a basic---yet powerful---implementation of speculation rules:
```html
```
### Prefetch
Speculation rules can also be used to just prefetch pages, without a full prerender. This can often be a good first step on the road to prerendering:
```html
```
> [!NOTE]
> **Note:** Unlike the older `` resource hint which just prefetched to the HTTP cache, documents loaded using speculation rules are processed in the same way that navigations are (but then not rendered) and are held in memory so will be available quicker to the browser once needed. This also allows documents which are not cacheable in the HTTP cache to be prefetched.
> [!CAUTION]
> Due to [issues with prefetching documents](https://github.com/whatwg/html/issues/6723) using ``, it is recommended to use speculation rules for navigations going forward, with `` only used for prefetching subresources. On the reverse side, speculation rules should be used for document navigation speculations and not for subresource prefetches, particularly given cross-origin restrictions of the Speculation Rules API.
### Prerender until script
> [!WARNING]
> **Experimental:** This is a new addition to the Speculation Rules API under development and not yet available by default. We are running [an origin trial](https://developer.chrome.com/blog/prerender-until-script-origin-trial.md.txt) for this feature from January 2026.
The Chrome team is also working on adding [`prerender_until_script`](https://github.com/WICG/nav-speculation/blob/main/prerender-until-script.md) to the Speculation Rules API (See: [implementation bug](https://crbug.com/428500219)). This would be a step between prefetch and prerender and would be used in a similar manner:
```html
```
Similar to [`NoState prefetch`](https://developer.chrome.com/blog/nostate-prefetch.md.txt) this would prefetch both the HTML document, and also the subresources available in that HTML. However it would go further than that and also start to prerender the page, stopping when the first script is encountered.
This would mean for pages with no JavaScript, or only JavaScript in the footer, the page could be almost fully prerendered in advance. Pages with scripts in the `` wouldn't be able to prerender but would still benefit from the subresource fetching.
This would avoid the risks of unintended side effects from JavaScript executing, but allow a much larger performance gain than just `prefetch`.
### Chrome limits
Chrome has limits in place to prevent overuse of the Speculation Rules API:
| eagerness | Prefetch | Prerender |
|---|---|---|
| `immediate` | 50 | 10 |
| `eager` / `moderate` on mobile | 6 (FIFO) | 2 (FIFO) |
| `eager` / `moderate` on desktop | 2 (FIFO) | 2 (FIFO) |
| `conservative` | 2 (FIFO) | 2 (FIFO) |
Speculation limits in Chrome.
The `eager`, `moderate` and `conservative` settings---which depend on user interaction---work in a [First In, First Out (FIFO) manner](https://en.wikipedia.org/wiki/FIFO_(computing_and_electronics)): after reaching the limit, a new speculation will cause the oldest speculation to be canceled and replaced by the newer one to conserve memory.
A canceled speculation can be triggered again---for example by hovering over that link again---which will result in that URL being re-speculated, pushing out the oldest speculation. In this case the previous speculation will have cached any cacheable resources in the HTTP Cache for that URL so speculationing a further time should have a reduced cost. This is why the limit is set to the modest threshold of 2 by default. We use a higher threshold of 6 for prefetch on mobile as there is no "hover", the alternative mobile viewport heuristics are less accurate, and prefetch is less costly than prerender (we keep the 2 limit for prerender on mobile).
Static list rules are not triggered by a user action and so have a higher limit as it is not possible for the browser to know which are needed and when they are needed.
Chrome is also [considering increasing the limit to 5 on mobile for `eager` and `moderate` at least for prefetch](https://issues.chromium.org/issues/443231314), given this is a less precise heuristic. Developers using `eager` may want to consider using `conservative` rules in addition for when multiple links are available in the viewport so that they still speculate even if not selected as one of the first two links.
The `immediate` limit is also dynamic, so removing a `list` URL script element will create capacity by canceling those removed speculations.
Chrome will also [prevent speculations being used in certain conditions](https://source.chromium.org/chromium/chromium/src/+/main:content/public/browser/preloading.h?q=PreloadingEligibility&ss=chromium) including:
- [Save-Data](https://web.dev/articles/optimizing-content-efficiency-save-data).
- [Energy saver](https://developer.chrome.com/blog/memory-and-energy-saver-mode.md.txt) when enabled and on low battery.
- Memory constraints.
- When the "Preload pages" setting is turned off (which is also explicitly turned off by Chrome extensions such as uBlock Origin).
- Pages opened in background tabs.
Chrome also does not render cross-origin iframes on prerendered pages until activation.
All of these conditions aim to reduce the impact of over-speculation when it would be detrimental to users.
### How to include speculation rules on a page
> [!TIP]
> **Tip:** See the [Guide to implementing speculation rules for more complex sites](https://developer.chrome.com/docs/web-platform/implementing-speculation-rules.md.txt) for more information on how to use speculation rules for more complex sites.
Speculation rules can be statically included in the page's HTML or dynamically inserted into the page by JavaScript:
- **Statically included speculation rules** : For example a news media site, or a blog may prerender the newest article, if that is often the next navigation for a large proportion of users, Alternatively, document rules with a `moderate` or `conservative` can be used to speculate as users interact with links.
- **Dynamically inserted speculation rules**: This could be based on application logic, personalized to the user, or based on other heuristics.
Those favoring dynamic insertion based on actions such as hovering over, or clicking down on a link---as many libraries have done in the past with ``---are recommended to look at document rules, as these allow the browser to handle many of your use cases.
Speculation rules can be added in either the `` or the `` of in the main frame. Speculation rules in subframes are not acted upon, and speculation rules in prerendered pages are only acted upon once that page is activated.
### The `Speculation-Rules` HTTP header
Browser Support
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[Source](https://developer.mozilla.org/docs/Web/HTTP/Reference/Headers/Speculation-Rules)
Speculation rules can also be delivered by using a `Speculation-Rules` HTTP header, rather than including them directly in the document's HTML. This allows easier deployment by CDNs without the need to alter document contents themselves.
The `Speculation-Rules` HTTP header is returned with the document, and points to a location of a JSON file containing the speculation rules:
```console
Speculation-Rules: "/speculationrules.json"
```
> [!WARNING]
> **Warning:** As this is a structured header, the quotes are important and **must** be included in the header value.
This resource must use the correct MIME type and, if it is a cross-origin resource, pass a CORS check.
```console
Content-Type: application/speculationrules+json
Access-Control-Allow-Origin: *
```
If you want to use relative URLs you may want to include the `"relative_to": "document"` key in your speculation rules. Otherwise, relative URLs will be relative to the speculation rules JSON file's URL. This may be particularly useful if you need to select some---or all---same-origin links.
### Speculation rules tag field
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[Source](https://developer.mozilla.org/docs/Web/HTML/Reference/Elements/script/type/speculationrules#tag)
It is also possible to add "tags" in the speculation rules JSON syntax at the overall level for all speculation rules in a ruleset:
```json
{
"tag": "my-rules",
"prefetch": [{
"urls": ["next.html"]
}],
"prerender": [{
"urls": ["next2.html"]
}]
}
```
Or at either the individual rule level:
```json
{
"prefetch": [{
"tag": "my-prefetch-rules",
"urls": ["next.html"]
}],
"prerender": [{
"tag": "my-prerender-rules",
"urls": ["next2.html"]
}]
}
```
This tag is then reflected in the `Sec-Speculation-Tags` HTTP header, which can be used to filter speculation rules at the server. The `Sec-Speculation-Tags` HTTP header can include multiple tags if the speculation is covered by multiple rules, as the following example show:
```console
Sec-Speculation-Tags: null
Sec-Speculation-Tags: null, "cdn-prefetch"
Sec-Speculation-Tags: "my-prefetch-rules"
Sec-Speculation-Tags: "my-prefetch-rules", "my-rules", "cdn-prefetch"
```
Some CDNs automatically inject speculation rules but block speculations for non-edge cached pages to avoid this feature resulting in increased origin-server usage. Tags allow them to identify speculations initiated by their default ruleset, but still allow any rules added by the site to pass through to the origin.
Ruleset tags are also [displayed in Chrome DevTools](https://developer.chrome.com/blog/new-in-devtools-137.md.txt#speculation-rule-tags).
### Speculation rules `target_hint` field
Browser Support
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[Source](https://developer.mozilla.org/docs/Web/HTML/Reference/Elements/script/type/speculationrules#target_hint)
Speculation rules can also include a `target_hint` field, which contains a [valid browsing context name or keyword](https://html.spec.whatwg.org/#valid-browsing-context-name-or-keyword) indicating where the page expects the prerendered content to be activated:
```html
```
This hint allows prerender speculations to be handled for `target="_blank"` links:
```html
Open this link in a new tab
```
For now only `"target_hint": "_blank"` and `"target_hint": "_self"` (the default if not specified) are supported in Chrome and only for prerender---prefetch is not supported.
`target_hint` is only needed for `urls` speculation rules, as for document rules the `target` is known from the link itself.
### Speculation rules and SPAs
Speculation rules are only supported for full page navigations managed by the browser, and not for Single Page Apps (SPA) or [app shell](https://developer.chrome.com/blog/app-shell.md.txt) pages. These architecture don't use document fetches, but instead make API or partial fetches of data or pages---which are then processed and presented in the current page. The data needed for these so called "soft navigations" can be prefetched by the app outside of speculation rules, but they cannot be prerendered.
Speculation Rules can be used to prerender the application itself from a previous page. This can help offset some of the extra initial load costs some SPAs have. However, route changes within the app cannot be prerendered.
### Debug speculation rules
See the [dedicated post on debugging speculation rules](https://developer.chrome.com/docs/devtools/application/debugging-speculation-rules.md.txt), for new Chrome DevTools features to assist with viewing and debugging this new API.
### Multiple speculation rules
Multiple speculation rules can also be added to the same page, and they append to the existing rules. Therefore, the following different ways all result in both `one.html` and `two.html` prerendering:
**List of URLs:**
```html
```
**Multiple `speculationrules` scripts:**
```html
```
**Multiple lists within one set of `speculationrules`**
```html
```
### `No-Vary-Search` support
Browser Support
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- ' xlink:href='%23s-b'%3E%3Cstop offset='0' stop-color='%2324a5f3' stop-opacity='0' /%3E%3Cstop offset='1' stop-color='%231e8ceb' /%3E%3C/radialGradient%3E%3CradialGradient id='s-j' cx='109.3' cy='13.8' r='93.1' gradientTransform='matrix(-.02 1.1 -1.04 -.02 137 -115)' xlink:href='%23s-b'%3E%3Cstop offset='0' stop-opacity='0' /%3E%3Cstop offset='1' stop-color='%235488d6' stop-opacity='0' /%3E%3Cstop offset='1' stop-color='%235d96eb' /%3E%3C/radialGradient%3E%3C/defs%3E%3Crect width='220' height='220' x='22' y='-107' fill='url(%23s-a)' ry='49' transform='matrix(.57 0 0 .57 187 256)' /%3E%3Cg transform='translate(194 190)'%3E%3Ccircle cx='67.8' cy='67.7' fill='url(%23s-c)' paint-order='stroke fill markers' r='54' /%3E%3Ccircle cx='-69.9' cy='69.3' fill='url(%23s-i)' transform='translate(138 -2)' r='54' /%3E%3C/g%3E%3Cellipse cx='120' cy='14.2' fill='url(%23s-j)' rx='93.1' ry='93.7' transform='matrix(.58 0 0 .58 192 250)' /%3E%3Cg transform='matrix(.58 0 0 .57 197 182)'%3E%3Cpath fill='%23cac7c8' d='M46 192h1l72-48-7-9-66 57Z' /%3E%3Cpath fill='%23fbfffc' d='M46 191v1l66-57-7-9-59 65Z' /%3E%3Cpath fill='url(%23s-d)' d='m119 144-7-9 66-57-59 66Z' /%3E%3Cpath fill='%23fb645c' d='m105 126 7 9 66-57-1-1-72 49Z' /%3E%3C/g%3E%3Cpath stroke='%23fff' stroke-linecap='round' stroke-miterlimit='1' stroke-width='1.3' d='m287 278 3-2m-12-17 8-2m-8-3h4m-4-13 8 2m-8 3h4m-1-13 7 3m-4-11 7 4m-2-11 6 6m0-12 6 7m1-11 4 6m4-10 3 7m5-9 2 7m15-7-1 7m10-5-3 7m11-4-4 7m11-2-5 6m16 7-7 4m10 4-7 3m10 6-8 1m8 16-8-2m5 10-7-3m4 11-7-4m2 11-6-5m0 11-5-6m-2 11-4-7m-4 11-3-8m-6 10-1-8m-16 8 2-8m-10 5 3-7m-11 4 4-7m-11 2 5-6m-8 3 3-3m4 8 2-3m5 8 2-4m6 7 1-4m8 5v-4m8 4v-4m9 3-1-4m9 1-2-4m9 0-2-4m9-2-3-3m8-4-3-2m8-5-4-2m7-6-4-1m5-8h-4m4-8h-4m3-9-4 1m1-9-4 2m-1-9-3 2m-2-9-3 3m-4-8-2 3m-5-8-2 4m-6-6-1 3m-8-5v4m-8-4v4m-9-2 1 3m-9 0 2 3m-9 1 2 3m-9 2 3 3m-8 4 3 2m-8 5 4 2m-7 6 4 1m-4 25 4-1m-2 5 7-3m-6 7 4-2m-2 6 7-4m-13-21h8m41-41v-8m0 99v-8m49-42h-8' transform='translate(-65 8)' /%3E%3C/svg%3E)
[Source](https://developer.mozilla.org/docs/Web/HTTP/Reference/Headers/No-Vary-Search)
When prefetching or prerendering a page, certain URL parameters (technically known as *search parameters*) may be unimportant to the page actually delivered by the server, and only used by client side JavaScript.
For example, [UTM parameters](https://en.wikipedia.org/wiki/UTM_parameters) are used by Google Analytics for campaign measurement, but usually don't result in different pages being delivered from the server. This means that `page1.html?utm_content=123` and `page1.html?utm_content=456` will deliver the same page from the server, so the same page can be reused from the cache.
Similarly, applications may use other URL parameters that are only handled client side.
The [No-Vary-Search](https://httpwg.org/http-extensions/draft-ietf-httpbis-no-vary-search.html) proposal allows a server to specify parameters that don't result in a difference to the resource delivered, and therefore allow a browser to reuse previously cached versions of a document which only differ by these parameters. This is supported in Chrome (and Chromium-based browsers) for navigation speculations for both prefetch and prerender.
> [!CAUTION]
> **Caution:** When using prerender with No-Vary-Search, the page may be preprendered with a different search parameter (or with no search parameter at all). Initially the URL (as returned by `location.href`, for example) will be the prerendered URL, but when a link is clicked with different search parameters covered by No-Vary-Search, the browser will use that prerendered page and replace the URL with the final URL (including the correct search parameters).
>
> Therefore, any JavaScript that depends on the search parameters should only be executed after the page is activated. For UTM parameters, this may not be a problem if analytics code does not run until after activation, but for client-side rendered pages that render different content based on search parameters, this can be a problem and caution should be used to ensure the client-side rendered content is updated on activation.
Speculation rules support using `expects_no_vary_search` to indicate where a `No-Vary-Search` HTTP header is expected to be returned. Doing so can help further avoid unnecessary downloads before the responses are seen.
```html
Product 123
Product 124
```
In this example, the `/products` initial page HTML is the same for both product IDs `123` and `124`. However, the page contents eventually differ based on client-side rendering using JavaScript to fetch product data using the `id` search parameter. So we prefetch that URL eagerly and it should return a `No-Vary-Search` HTTP header showing the page can be used for any `id` search param.
However, if the user clicks on any of the links before the prefetch has completed, the browser may not have received the `/products` page. In this case, the browser does not know if it will contain the `No-Vary-Search` HTTP header. The browser is then left with a choice of whether to fetch the link again, or wait for the prefetch to complete to see if it contains a `No-Vary-Search` HTTP header. The `expects_no_vary_search` setting allows the browser to know the page response is expected to contain a `No-Vary-Search` HTTP header, and to wait for that prefetch to complete.
> [!NOTE]
> **Note:** Using just `expects_no_vary_search` is not enough to ensure a page with a different search parameter is reused---it is more a hint to the browser about what to expect. The server must also return a `No-Vary-Search` HTTP header for the browser to know it can use the page from its cache even when the search parameter does not match.
You can also add multiple parameters to `expects_no_vary_search` with a space separating them (since `No-Vary-Search` is an HTTP structured header):
```json
"expects_no_vary_search": "params=(\"param1\" \"param2\" \"param3\")"
```
### Speculation rules restrictions and future enhancements
Speculation rules are restricted to pages opened within the same tab, but [we are working to reduce that restriction](https://issues.chromium.org/issues/40234240).
By default, prerenders are restricted to same-origin pages. Prerendering same-site cross-origin pages (for example, `https://a.example.com` could prerender a page on `https://b.example.com`) is possible to enable. To use this the speculated page (`https://b.example.com` in this example) needs to opt-in by including a `Supports-Loading-Mode: credentialed-prerender` HTTP header or Chrome will cancel the speculation.
Future versions may also [allow prerender for non-same-site, cross-origin pages](https://issues.chromium.org/issues/40168192) as long as cookies don't exist for the prerendered page and the prerendered page opts in with a similar `Supports-Loading-Mode: uncredentialed-prerender` HTTP header.
Speculation rules do already support cross-origin prefetches, For same-site, cross-origin prefetches there are no restrictions. Different-site, cross-origin prefetches are only supported when cookies for the cross-origin domain don't exist. If cookies exist from the user having visited that site before, then the speculation won't be used and will show a failure in DevTools.
> [!NOTE]
> **Note:** Chrome will still send an anonymous prefetch request (without cookies) if [the private prefetch proxy is used through the `anonymous-client-ip-when-cross-origin` directive](https://developer.chrome.com/blog/search-speculation-rules.md.txt#privacy-preserving-prefetches), but the prefetch response will be discarded. This prevents the page and proxy together from being able to discover if the user has cookies for a third-party site. When the proxy is not used, no prefetch is sent when cookies exist.
Given those current limitations, one pattern that can improve your users experiences for both internal links and external links where possible is to prerender same-origin URLs and attempt to prefetch cross-origin URLs:
```html
```
The restriction to prevent cross-origin speculations for cross-origin links by default is necessary for security. It is an improvement over `` for cross-origin destinations which will also not send cookies but still attempt the prefetch---which will be either result in a wasted prefetch that needs to be resent or, worse still, the incorrect page loading.
### Detect Speculation Rules API support
You can feature detect Speculation Rules API support with standard HTML checks:
```javascript
if (HTMLScriptElement.supports && HTMLScriptElement.supports('speculationrules')) {
console.log('Your browser supports the Speculation Rules API.');
}
```
### Add speculation rules dynamically through JavaScript
This is an example of adding a `prerender` speculation rule with JavaScript:
```javascript
if (HTMLScriptElement.supports &&
HTMLScriptElement.supports('speculationrules')) {
const specScript = document.createElement('script');
specScript.type = 'speculationrules';
specRules = {
prerender: [
{
urls: ['/next.html'],
},
],
};
specScript.textContent = JSON.stringify(specRules);
console.log('added speculation rules to: next.html');
document.body.append(specScript);
}
```
You can view a demo of Speculation Rules API prerendering, using JavaScript insertion, on this [prerender demo page](https://chrome.dev/prerender-demos/).
Inserting a `
```
You can change this to a dynamically inserted script element which only inserts based on the previous `whenActivated` function:
```javascript
async function addScript(scriptUrl) {
await whenActivated;
const script = document.createElement('script');
script.src = 'scriptUrl';
document.body.appendChild(script);
}
addScript('https://example.com/app/script.js');
```
This can be useful to hold back distinct scripts that include analytics, or render content based on state or other variables that can change during the span of a visit. For example, recommendations, or login state, or shopping basket icons could all be held back to ensure the most up to date information is presented.
While this is perhaps more likely to happen more often with the use of prerendering, these conditions are also true for pages loaded in background tabs mentioned previously (so the `whenFirstVisible` function could be used in place of `whenActivated`).
In many cases state should ideally also be checked on general `visibilitychange` changes---for example, when returning to a page that has been background, any shopping basket counters should be updated with the latest number of items in the basket. So this is not a prerender-specific problem but prerender is just making an existing issue more obvious.
One way that Chrome mitigates some of the need for manually wrapping scripts or functions, is that [certain APIs are held back as mentioned previously](https://developer.chrome.com/docs/web-platform/prerender-pages.md.txt#apis-heldback), and also third-party iframes are not rendered, so it's only content on top of this that is required to be manually held back.
### Measure performance
For measuring performance metrics, analytics should consider whether it is better to measure these based upon the activation time rather than the page load time that browser APIs will report.
For Core Web Vitals, measured by Chrome through the [Chrome User Experience Report](https://developer.chrome.com/docs/crux.md.txt), these are intended to measure the user experience. So these are measured based on activation time. This will often result in a 0 second LCP for example, showing this is great way of improving your Core Web Vitals.
From version 3.1.0, the [`web-vitals` library](https://github.com/GoogleChrome/web-vitals) has been updated to handle prerendered navigations in the same way Chrome measures Core Web Vitals. This version also flags prerendered navigations for those metrics in the [`Metric.navigationType`](https://github.com/GoogleChrome/web-vitals#metric) attribute if the page was fully or partially prerendered.
### Measure prerenders
Whether a page is prerendered can be seen with a non-zero `activationStart` entry of [`PerformanceNavigationTiming`](https://developer.mozilla.org/docs/Web/API/PerformanceNavigationTiming). This can then be logged using a Custom Dimension, or similar when logging the page views, for example using the [`pagePrerendered` function described previously](https://developer.chrome.com/docs/web-platform/prerender-pages.md.txt#detect-prerender-in-javascript):
```javascript
// Set Custom Dimension for Prerender status
gtag('set', { 'dimension1': pagePrerendered() });
// Initialise GA - including sending page view by default
gtag('config', 'G-12345678-1');
```
This will allow your analytics to show how many navigation are prerendered compared to other types of navigation, and also allow you to correlation any performance metrics or business metrics to these different navigation types. Faster pages means happier users, which can often have real impact on business measures as our [case studies](https://web.dev/case-studies) show.
As you measure the business impact of prerendering pages for instant navigations, you can decide whether it is worth investing more effort in using this technology to allow more navigations to be prerendered, or to investigate why pages are not being prerendered.
#### Measure hit rates
In addition to measuring the impact of pages that are visited after a prerender, it is also important to measure pages that are prerendered and *not* subsequently visited. This could imply you are prerendering too much, and using up valuable resources of the user for little benefit.
This can be measured by firing an analytics event when speculation rules are inserted---after checking the browser supports prerendering using `HTMLScriptElement.supports('speculationrules')`---to indicate that prerender was requested. (Note that just because a prerender was requested, does not indicate that a prerender was started or completed as, as noted previously, a prerender is a hint to the browser and it may choose not to prerender pages on user settings, current memory usage, or other heuristics.)
You can then compare the number of these events, to the actual prerender page views. Or alternatively fire another event on activation if that makes it easier to compare.
The "successful hit rate" can then be approximated by looking at the difference between these two figures. For pages where you are using the Speculation Rules API to prerender the pages, you can adjust the rules appropriately to ensure you keep a high hit rate to maintain the balance between using up the users resources to help them, versus using it needlessly.
Be aware that some prerendering may be taking place due to the address bar prerendering and not just your speculation rules. You can check the `document.referrer` (which will be blank for address bar navigation including prerendered address bar navigations) if you want to differentiate these.
Remember to also look at pages which have no prerenders, as that could indicate these pages are not eligible for prerendering, even from the address bar. That may mean you are not benefiting from this performance enhancement. The Chrome team is looking to add extra tooling to test for Prerender eligibility perhaps [similar to the bfcache testing tool](https://web.dev/articles/bfcache#test_to_ensure_your_pages_are_cacheable), and also potentially add an API to expose why a prerender failed.
## Impact on extensions
See the dedicated post on [Chrome Extensions: Extending API to support Instant Navigation](https://developer.chrome.com/blog/extension-instantnav.md.txt) which details some additional considerations extension authors may need to think about for prerendered pages.
## Feedback
Prerendering is in active development by the Chrome team, and there are plenty of plans to expand the scope of what has been made available in the Chrome 108 release. We welcome any feedback on [the GitHub repo](https://github.com/WICG/nav-speculation/issues) or using [our issue tracker](https://issues.chromium.org/issues/new?component=1456902&template=0), and look forward to hearing and sharing case studies of this exciting new API.
## Related links
- [Speculation Rules Codelab](https://codelabs.developers.google.com/speculation-rules)
- [Debugging speculation rules](https://developer.chrome.com/docs/devtools/application/debugging-speculation-rules.md.txt)
- [Introducing NoState Prefetch](https://developer.chrome.com/blog/nostate-prefetch.md.txt)
- [Speculation Rules API specification](https://wicg.github.io/nav-speculation/speculation-rules.html)
- [The Navigational speculation GitHub repo](https://github.com/WICG/nav-speculation/)
- [Chrome Extensions: Extending API to support Instant Navigation](https://developer.chrome.com/blog/extension-instantnav.md.txt)
## Acknowledgements
*Thumbnail image by [Marc-Olivier Jodoin](https://unsplash.com/@marcojodoin) on [Unsplash](https://unsplash.com/photos/NqOInJ-ttqM)*