[Webkit-unassigned] [Bug 208482] New: SVG and Canvas content should factor into visually-non-empty heuristic

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=208482

            Bug ID: 208482
           Summary: SVG and Canvas content should factor into
                    visually-non-empty heuristic
           Product: WebKit
           Version: WebKit Local Build
          Hardware: Unspecified
                OS: Unspecified
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: Layout and Rendering
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: nham at apple.com
                CC: bfulgham at webkit.org, simon.fraser at apple.com,
                    zalan at apple.com

Currently, we block the initial render on our "visually non-empty" heuristic. This heuristic depends on (among other things) how much text is in the document, how many image pixels are in the document, and whether or not the main resource is fully parsed. However, SVG and canvas content is not considered by the heuristic.

This causes a delayed first contentful paint for certain websites. For instance, YouTube's first contentful paint is often delayed until far after the main HTML resource is parsed:

1. They don't ship down any text initially, so we won't trip over the visually non-empty character threshold
2. They don't ship down any actual images initially, so we won't trip over the filled-image-pixel threshold (everything that looks like an image is actually an SVG)
3. On an uncached load, we won't paint after parsing the initial HTML either, since there's usually an outstanding web font loading, and our heuristic delays the first paint until the web fonts load if there isn't enough character or image content

We could improve our FCP for YT and other similar sites by including an estimate of SVG and canvas content sizes in the visually-non-empty heuristic. This will also improve FCP as reported by https://w3c.github.io/paint-timing/#first-contentful-paint.

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