Define purpose of other namespaced markup in <title> and <desc>

[nikosandronikos]

https://svgwg.org/svg2-draft/struct.html#issue23

Prior discussion, from https://www.w3.org/2015/03/05-svg-minutes.html#item12:

ed: I think the issue is to say what this purpose is

heycam: presumably to include some more structured descriptions rather than just plain text

AmeliaBR: and originally also so that you could have complex fallback content, in a browser that didn't support SVG
... that's no longer relevant though
... I'm pretty sure the main purpose is so you can have structured content inside <desc>

heycam: note also the HTML parsing algorithm allows HTML content inside here

AmeliaBR: I'm not sure if there's much of a benefit within <title>, but some browsers do support <br> within <title> and some don't
... a weird random thing
... maybe that should be cleaned up

heycam: within SVG documents or HTML title?

http://stackoverflow.com/questions/23626422/tooltip-to-display-in-two-separate-lines-not-working-in-firefox/25690802#25690802

AmeliaBR: so this may have been coming from this comment that external namespaces are OK
... so some support <br> within title

Tav: I don't really like the idea of having <br> in there
... maybe white-space:pre-line?

AmeliaBR: that's the other thing that's changed since SVG 1.1
... in the title="" in HTML you can put line breaks
... so either way isn't conforming to specs, but all browsers allow one way or the other
... either with <br> or a hard line break

heycam: and that's a hard line break without setting the white-space property?

AmeliaBR: yes
... and only for <title>

Tav: putting a <br> seems wrong to me
... allowing white space to be preserved seems more compatible

AmeliaBR: and with the new white-space option, I think that will be more reasonable

Tav: you can style your title

AmeliaBR: it's tricky, because tooltips aren't styled with CSS
... I have no idea whether white-space:pre would apply

heycam: do you think we need a issue in here to define this kind of thing?

[nikosandronikos]

Discussion: https://www.w3.org/2016/04/20-svg-minutes.html#item04

[nikosandronikos]

For SVG 2, we can leave it as allowed, but undefined.

So authors can put HTML or whatever inside title and desc and it won't break anything, but it won't have a defined beahviour.

[jarek-foksa]

@nikosandronikos isn't this what <metadata> element should be used for? It's even allowed in the same places where <title> and <desc> can appear.

I would rather keep it simple and allow only character data inside <title> and <desc>.