Notify users of links that open in new tab.

[wmalexander]

I had a conversation with our Technology Accessibility Team last week - they were asking me if we could implement an automated icon addition for links which open in an external tab. Apparently there are some WCAG compliance implications of opening a link in a new tab.
​https://theadminbar.com/accessibility-weekly/opening-links-in-new-tabs-or-not/

This may be the responsibility of the theme, rather than being a core WordPress function. I thought it was worth mentioning, since it came up.

[sabernhardt]

Hi and thanks for the ticket! Most related Trac tickets have focused on the admin and on removing target="_blank".

GB61056 requested a class for the "Button" block so the theme's visual indicator could show in the editor as it would on the front end. GB22332 could add screen reader text for a non-visual indicator.

I set the component to Themes. Even if Core creates a common method of indicating links that open in a new tab, themes likely will need to opt in (with something like add_theme_support).

[joedolson]

This does seem mostly like a theme issue, since it's about how the content displays. But I also feel that it can be a difficult thing to add with back compatibility, since people who have been concerned about this issue will have already dealt with it - either using plugins or by adding explicit text to their link text to indicate that the link opens in a new tab.

It's not impossible that this could be done in core, but it's very similar to the reason why we decided not to output the .screen-reader-text styles in themes. See #29699.