From the discussion linked in the Webkit repo:

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Philip: First, google slides is written in svg, so that won't change with this. But google docs is using canvas, so they might be a candidate. … they might want to integrate this peicemeal, this API allows them to start to adopt the feature slowly,

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This reads to me like "Google Docs decided to go with canvas sometime ago [1], found it to be too hard, so pushed Chrome to have a way to support HTML in Canvas. The rest is just post-hoc justifications"

[1] https://workspaceupdates.googleblog.com/2021/05/Google-Docs-...