Reproduction

1. Create a page.
2. Insert a video block.
3. Save and then watch the HTML output in the frontend.

Actual

• The <video> element does not have the attributes width and height.
• This causes cumulative layout shift (CLS) which is bad for usability and SEO metrics.
  • Either during DOM initialization — the element starts with dimensions guessed from its CSS width plus the assumed fallback aspect ratio of a video (e.g. in Safari this seems to be 2:1).
  • Or if the video uses preload = none, also after the DOM is ready, it has this provisional dimensions and as soon as the user clicks PLAY, and the browser starts fetching the video and determines its width/height, then layout shift occurs.

Expected

1. When you insert a video block and set/change the source,
2. … the Block Editor gets the image dimensions from:
   • a. Supported video service
     • ✅ YouTube — Creates an iframe with concrete width/height, was seemingly done in Can't style embedded videos properly due to inline width & height #8383
     • ❓Vimeo — Did not test this.
   • b. ❌ The Media Library — Tested: Creates no width/height attributes.
   • c. ❓ Externally hosted video file — Did not test this yet — By an API which returns the video dimensions by loading the video's first few bytes (moov atom at the start) or the last bytes (metadata at the end)
3. … and inserts the width and height parameters into the video element accordingly:
   • Independent from the block dimension or alignment settings ( None, Wide, Full Width). Like with the Image block with its own dimensions and the underlying img element with its src, width, height.
   • And does this for each source as soon as we have Video Block: alternate sources such as ogv and webm file formats #9457

Environment

• WordPress 6.2.2
• Gutenberg 15.9.1

Infos to be amended by experts please

• Prioritization: I assume CLS optimization is a goal of WordPress.
  • I assume this is not a SEO luxury only.
  • But a real usability problem (especially when having multiple videos (with aspect ratios quite different than the assumed fallback)) — Then viewport shifting is noticeable to even disturbing.
• WordPress Classic: Does it create video elements with width and height?
  • I never used Classic, and do not plan to set it up.
  • If someone knows please comment.
• I guess what I describe under "expected" steps 2abc probably all need certain WordPress Core API calls.
  • Please tell whether such functions exists or not in comments below.
  • Please file corresponding ticket(s) in WP Core to provide the necessary function(s).