The "Accessibility" tab in the Info panel checks your site for accessibility issues. Because checking a site is a relatively heavy operation, polypane does not run a check automatically but you'll have to start it by clicking the 'Analyse page for accessibility issues'.
Polypane will show a list of issues ordered by severity and highlight any matching WCAG criteria. Click them open to show which elements are affected by an issue and for the issue specific for that element.
When you hover over the element's HTML content, the element will be scrolled to and highlighted in all panes.
Clicking the 'inspect element' editor will show the element in the elements panel so you can quickly debug the issue, then run the accessibility check again (with the reload button in the panel) to check you've solved the issue.
Polypane is filled with accessibility features. We wrote a full overview on our blog: Find and fix accessibility issues with Polypane.
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