There are different solutions for this task, and I know these three:
All of these repositions appear to consist mainly of some JavaScript and CSS files. The first two repositories have a link to some demo page on which I took pictures.
The first one provides a Greasemonkey / Tampermonkey script that allows you to enter this overlay on any suitable hocr website online and local (some configuration may be possible for this). I don't know how difficult it is to use other solutins for your own hocr files, but this should be doable.
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