On some sites (e.g. github.com) bookmarklet does not work because of Content Security Policy. But with chrome extension it's possible to inject DiagnostiCSS into any page!
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Download diagnosticss.crx file
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Settings icon > Tools > Extensions (The three horizontal bars icon in the top-right corner)
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Drag and drop the "crx" extension file onto the Extensions page from step 1 (.crx file should likely be in your Downloads directory)
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Install.