Google’s John Mueller said on Twitter that Google ignores capitalization in HTML tag attributes. For example, some frameworks show a warning if the hrefLang is written in lowercase: hreflang. Google does not care about this:
You mean the capitalization? IMO capitalization in HTML attribute names is ignored (eg in XHTML they're often completely in caps). Personally, I'd try to be consistent, but unless you're using tools of your own that are super picky, it probably doesn't matter.
— ? John ? (@JohnMu) October 29, 2020
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