ensure that "readability backplate" is not a fingerprinting vector (Windows High Contrast mode)
From #33534 (closed): Added in Firefox 73: in Windows' High Contrast Mode, web renderer now adds 'readability backplate' of solid color between background and text. Is this a fingerprinting vector?
https://bugzilla.mozilla.org/show_bug.cgi?id=1539212
"implement readability backplate for high contrast mode"
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