I got this working in [my prototype](https://github.com/jcsteh/firefox/tree/wip/genTaggedPdf), but it required a small patch to Skia and I'm not sure it will be accepted upstream. I suspect fixing Gecko to provide text instead of just glyph indexes might be a much bigger lift, but I'm not familiar enough with the graphics code to have any real clue.
Bug 1950656 Comment 4 Edit History
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I got this working in [my prototype](https://github.com/jcsteh/firefox/tree/wip/genTaggedPdf), but it required a small patch to Skia and I'm not sure it will be accepted upstream. I suspect the cleaner solution (as far as Skia is concerned) of fixing Gecko to provide text instead of just glyph indexes might be a much bigger lift, but I'm not familiar enough with the graphics code to have any real clue.