I've been asked to make a site fully accessible. I had no problem reaching WCAG Priority 2, but priority 3 requires that all links (and forms etc) have tabindex set. Whilst I can do this easily for any of the links built in to the theme, I'm not sure how to do this for user generated links in posts. I figure that I need to change something in wordpress core so that when you add a link it adds tabindex="x" next to the href and auto-increments, but I'm not sure which file I'd need to edit. Ideally I'd prefer not to change anything in the core files at all so that I don't have to do it again next time there's an update so - any suggestions?