The three images in the illustration below are all from the same source image.
Validating opacity css
PS: I hope you don't mind my changing of the issue's title.
I think now it's easier to locate and will prevent duplicates.
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Either your document contains characters disallowed by XML 1.0 (see Section 2.2 of XML 1.0 for details) or your document isn't encoded as expected.
This validator checks the markup validity of Web documents in HTML, XHTML, SMIL, Math ML, etc.