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I used The W3C CSS Validation Service here (http://jigsaw.w3.org/css-validator/) to check the following webpage:
https://cnx.org/contents/i4nRcikn@2.89:Ib8OK9lb@4/Inverse-Functions
and the document validates as CSS level 3 + SVG.
I also used colorfilter to check the colors of the same page here:
https://www.toptal.com/designers/colorfilter
and the colors work for all the filters.
I personally really like how this textbook is designed and written. It's great to know that the developers have inclusive design in mind.
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