Quick and dirty testing of WCAG 2.1 Success Criterion 1.4.12 Text Spacing.
Experimental:use at own risk!
Bookmarklet: Text Spacing (drag to bookmarklets bar or right click and save as bookmark)
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Last updated 26th of March 2020. Now works on content inside same-origin first level iframes - i.e. it doesn't then work if there's an iframe inside another iframe. Thanks to Patrick H. Lauke