[CHANGE] External References (i.e. URLs)

[Rob Beezer]

For a year now, we've had an experimental @visual attribute on the "url"
element. When a "url" has content (in other words, you author text to become
the clickable) then the visual version of the URL automatically becomes a
footnote in a monospace font. This device works so well for non-electronic
formats that it will now be required.

When you do not provide content (which should be an exception) then @visual will
be optional, but will be used preferentially (over @href) as the clickable text.

So:

Best: <url href="http://example.com/" visual="example.com">Demo Site</url>

OK: <url href="http://example.com/" visual="example.com"/>

Poor: <url href="http://example.com/"/>

Deprecated: <url href="http://example.com/">Demo Site</url>

You will get warnings for the deprecated version. Yes, it will be a PITA to
edit all of those (tell me about it, my deprecation-pain is always greater than
yours). But they will continue to behave acceptably, as you will get @href in a
footnote. For now. Read documentation for more on rationale and finer points.

Section 4.26: URLs and External References
https://pretextbook.org/doc/guide/html/topic-url.html

This is important for your source to behave well for readers who cannot (or
choose not) to use electronic versions of output, specifically hardcopy print
and embossed braille. And if you use the "Best" version above routinely, your
narrative should read much more smoothly for all your readers in all formats.
(And isn't that the point?)

Rob