On the web, hitting Cmd/Ctrl-A (or triggering "Select All" another way) has two distinct modes of operation.
If your focus is inside a <textare...
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This is cool! Could be something to propose to the dev.to codebase (since you specifically call out having a large editor π).
Unfortunately it does not work in Firefox. :(
I've not had time to test this but I've had another report which was very excited to find out that it does:
Try Emojityperβtype some text/emoji, tab-focus to a button, and then try Select All.