This is reproducible in https://blurry-constraints.glitch.me/
<rdar://problem/116810370>
This is impacting us as well. Removing the width/height constraint is a workaround for now. It'll allow it to capture at original resolution.
Pull request: https://github.com/WebKit/WebKit/pull/19047
Committed 269406@main (2a1363e51a7c): <https://commits.webkit.org/269406@main> Reviewed commits have been landed. Closing PR #19047 and removing active labels.
*** Bug 258626 has been marked as a duplicate of this bug. ***