Allow the target attribute on <a> tags.

[pento]

Description

Sine we have UI for adding target="_blank" to <a> elements, we should allow it to stay when converting existing posts to blocks.

This PR partially address #4498.

How has this been tested?

Create a post in the Classic Editor with this content:

<a href="https://wordpress.org" target="_blank">Hi!</a>

Open the post in Gutenberg, and convert it to blocks.

Click on the link, check that the "Open in new window" setting is enabled.

Preview the post, check that clicking the link opens it in a new window.

Checklist:

• My code is tested.
• My code follows the WordPress code style
• My code follows the accessibility standards.
• My code has proper inline documentation.

[youknowriad]

We don't have the UI to add other kinds of targets (aside _blank). We also add rel="noreferrer noopener" to these links. The question is: Should we allow only target="_blank" and normalize the "rel" attribute in that case? or it's fine to just pick the target and keep the link as is?

[mcsf]

Those are good questions. Perhaps we should perform some normalization, as there is an expectation — given the precedents when pasting from word processors, etc. — that Gutenberg generally cleans up markup. I was looking at the raw-handling pipeline and am guessing that an acceptable place to normalize target and rel would be phrasing-content-reducer, next to where special rules are applied to b and i tags:

gutenberg/packages/blocks/src/api/raw-handling/phrasing-content-reducer.js

Lines 24 to 28 in 09db904

	} else if ( node.nodeName === 'B' ) {
	node = replaceTag( node, 'strong', doc );
	} else if ( node.nodeName === 'I' ) {
	node = replaceTag( node, 'em', doc );
	}

[pento]

Good idea, @mcsf, I added normalisation here.

[afercia]

Well as an accessibility person I'd very happy to see these links have also a visually hidden text (opens in a new window) together with rel="noopener noreferrer". However, I'd suggest to consider this is about the users content and WordPress shouldn't modify users content without their consent.

[youknowriad]

LGTM 👍

[ellatrix]

I would have liked to see this filter only on block conversion, not on paste. I'm not sure if the user intends these link to open in a new window if the source does.

[pento]

It might be a good idea to add some styling to links that are set to open in a new window, so the author can easily see which links are set to do that.