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And for it to show up in a search engine's results page without looking like a strange URL you'd associate with a file in a CMS media library (s3.aws.com, or wp-content/2022/04/01/image.jpg)

My mistake!

https://google.github.io/styleguide/htmlcssguide.html

Only on XHTML, actually.

Source? Seems to be required based on https://validator.w3.org/nu/ which is also linked as the validator to use for WHATWG from https://whatwg.org/validator/