Update Image Prioritizer docs

Author: westonruter

plugins/image-prioritizer/readme.txt

@@ -15,7 +15,7 @@ This plugin optimizes the loading of images (and videos) with prioritization to
 
 The current optimizations include:
 
-1. Add breakpoint-specific `fetchpriority=high` preload links (`LINK[rel=preload]`) for image URLs of LCP elements:
+1. Add breakpoint-specific `fetchpriority=high` preload links (both as `LINK[rel=preload]` elements and `Link` response headers) for image URLs of LCP elements:
    1. An `IMG` element, including the `srcset`/`sizes` attributes supplied as `imagesrcset`/`imagesizes` on the `LINK`.
    2. The first `SOURCE` element with a `type` attribute in a `PICTURE` element. (Art-directed `PICTURE` elements using media queries are not supported.)
    3. An element with a CSS `background-image` inline `style` attribute.

plugins/optimization-detective/docs/extensions.md

@@ -10,7 +10,7 @@ As mentioned above, this plugin is a dependency that doesn't provide features on
 
 **[Image Prioritizer](https://wordpress.org/plugins/image-prioritizer/) ([GitHub](https://github.com/WordPress/performance/tree/trunk/plugins/image-prioritizer)):**
 
-1. Add breakpoint-specific `fetchpriority=high` preload links (`LINK[rel=preload]`) for image URLs of LCP elements:
+1. Add breakpoint-specific `fetchpriority=high` preload links (both as `LINK[rel=preload]` elements and `Link` response headers) for image URLs of LCP elements:
     1. An `IMG` element, including the `srcset`/`sizes` attributes supplied as `imagesrcset`/`imagesizes` on the `LINK`. ([1](https://github.com/WordPress/performance/blob/f5f50f9179c26deadeef966734367d199ba6de6f/plugins/image-prioritizer/class-image-prioritizer-img-tag-visitor.php#L167-L177), [2](https://github.com/WordPress/performance/blob/f5f50f9179c26deadeef966734367d199ba6de6f/plugins/image-prioritizer/class-image-prioritizer-img-tag-visitor.php#L304-L349))
     2. The first `SOURCE` element with a `type` attribute in a `PICTURE` element. (Art-directed `PICTURE` elements using media queries are not supported.) ([1](https://github.com/WordPress/performance/blob/f5f50f9179c26deadeef966734367d199ba6de6f/plugins/image-prioritizer/class-image-prioritizer-img-tag-visitor.php#L192-L275), [2](https://github.com/WordPress/performance/blob/f5f50f9179c26deadeef966734367d199ba6de6f/plugins/image-prioritizer/class-image-prioritizer-img-tag-visitor.php#L304-L349))
     3. An element with a CSS `background-image` inline `style` attribute. ([1](https://github.com/WordPress/performance/blob/f5f50f9179c26deadeef966734367d199ba6de6f/plugins/image-prioritizer/class-image-prioritizer-background-image-styled-tag-visitor.php#L62-L92), [2](https://github.com/WordPress/performance/blob/f5f50f9179c26deadeef966734367d199ba6de6f/plugins/image-prioritizer/class-image-prioritizer-background-image-styled-tag-visitor.php#L182-L203))
@@ -22,7 +22,9 @@ As mentioned above, this plugin is a dependency that doesn't provide features on
     1. Apply lazy loading to `IMG` tags based on whether they appear in any breakpoint’s initial viewport. ([1](https://github.com/WordPress/performance/blob/f5f50f9179c26deadeef966734367d199ba6de6f/plugins/image-prioritizer/class-image-prioritizer-img-tag-visitor.php#L124-L133))
     2. Implement lazy loading of CSS background images added via inline `style` attributes. ([1](https://github.com/WordPress/performance/blob/f5f50f9179c26deadeef966734367d199ba6de6f/plugins/image-prioritizer/class-image-prioritizer-background-image-styled-tag-visitor.php#L205-L238), [2](https://github.com/WordPress/performance/blob/f5f50f9179c26deadeef966734367d199ba6de6f/plugins/image-prioritizer/helper.php#L365-L380), [3](https://github.com/WordPress/performance/blob/f5f50f9179c26deadeef966734367d199ba6de6f/plugins/image-prioritizer/lazy-load-bg-image.js))
     3. Lazy-load `VIDEO` tags by setting the appropriate attributes based on whether they appear in the initial viewport. If a `VIDEO` is the LCP element, it gets `preload=auto`; if it is in an initial viewport, the `preload=metadata` default is left; if it is not in an initial viewport, it gets `preload=none`. Lazy-loaded videos also get initial `preload`, `autoplay`, and `poster` attributes restored when the `VIDEO` is going to enter the viewport. ([1](https://github.com/WordPress/performance/blob/f5f50f9179c26deadeef966734367d199ba6de6f/plugins/image-prioritizer/class-image-prioritizer-video-tag-visitor.php#L163-L246), [2](https://github.com/WordPress/performance/blob/f5f50f9179c26deadeef966734367d199ba6de6f/plugins/image-prioritizer/helper.php#L365-L380), [3](https://github.com/WordPress/performance/blob/f5f50f9179c26deadeef966734367d199ba6de6f/plugins/image-prioritizer/lazy-load-video.js))
-5. Ensure that [`sizes=auto`](https://make.wordpress.org/core/2024/10/18/auto-sizes-for-lazy-loaded-images-in-wordpress-6-7/) is added to all lazy-loaded `IMG` elements. ([1](https://github.com/WordPress/performance/blob/f5f50f9179c26deadeef966734367d199ba6de6f/plugins/image-prioritizer/class-image-prioritizer-img-tag-visitor.php#L148-L163))
+5. Responsive image sizes:
+    1. Compute the `sizes` attribute using the widths of an image collected from URL Metrics for each breakpoint (when not lazy-loaded since then handled by `sizes=auto`). ([1](https://github.com/WordPress/performance/blob/6459571471b26aee4f63f00e2ba9dfe6f5ce2f39/plugins/image-prioritizer/class-image-prioritizer-img-tag-visitor.php#L170-L184), [2](https://github.com/WordPress/performance/blob/6459571471b26aee4f63f00e2ba9dfe6f5ce2f39/plugins/image-prioritizer/class-image-prioritizer-img-tag-visitor.php#L412-L444))
+    2. Ensure [`sizes=auto`](https://make.wordpress.org/core/2024/10/18/auto-sizes-for-lazy-loaded-images-in-wordpress-6-7/) is set on `IMG` tags after setting correct lazy-loading (above). ([1](https://github.com/WordPress/performance/blob/6459571471b26aee4f63f00e2ba9dfe6f5ce2f39/plugins/image-prioritizer/class-image-prioritizer-img-tag-visitor.php#L156-L168))
 6. Reduce the size of the `poster` image of a `VIDEO` from full size to the size appropriate for the maximum width of the video (on desktop). ([1](https://github.com/WordPress/performance/blob/f5f50f9179c26deadeef966734367d199ba6de6f/plugins/image-prioritizer/class-image-prioritizer-video-tag-visitor.php#L84-L125))
 
 **[Embed Optimizer](https://wordpress.org/plugins/embed-optimizer/) ([GitHub](https://github.com/WordPress/performance/tree/trunk/plugins/embed-optimizer)):**