Improvements for wp-signup.php and wp-activate.php markup and CSS

[afercia]

Splitting this out from #23197.

First time trying to customize the network registration screens and noticed a few issues.

Bugs that should be addressed, also related to accessibility:

• labels not correctly associated (see screenshot)
• mixed use of explicitly and implicitly associated labels: explicitly associated ones should be preferred
• text not wrapped in semantic elements, often just within <div>s
• an <ul> list (noemail-tips) output within a paragraph
• buttons have a very unique styling, pretty inconsistent with the default WP styles. They don't use the .button CSS class so loading the .buttons.css stylesheet won-t have any effect; the body element also would need a wp-core-ui CSS class
• indentation: spaces instead of tabs

Possible improvements, maybe to split out in separate tickets:

• I'm not sure to understand why wpmu_signup_stylesheet() and wpmu_activate_stylesheet() output inline styles, maybe it's just for historical reasons; ideally, deprecate them or move the styles to login.css
• wp-activate fails to output a proper document <title> tag, not sure this can be solved (see #23197)
• when customizing the registration/activation screens, it's very difficult to distinguish all the screens for styling purposes: there's $stage that can be used to some extent, for example to add CSS classes on the body but it doesn't cover all the cases. Ideally, there should be an easy way to add CSS classes for each step or they should be built-in

Two labels for the same element, the first one should be a fieldset legend together with the sentence "Allow search engines...":

Replying to afercia:

wp-activate fails to output a proper document <title> tag, not sure this can be solved (see #23197)

This was previously fixed in #13638 and #24960, looks like it was reintroduced at some point.

The first part related to markup and CSS could be ideal for a good first bug. Moving to future release.

converts spaces to tabs, extracts <ul> from within <p>

My patch is partial in nature. I wanted to clarify with @afercia if these were the buttons were the ones you were referencing (see screenshots below) that needed the button class added (with other alignment adjustments as well)

@allisonplus thanks! I was referring to the buttons in the signup form, the ones you get on a multisite installation when you register on the front-end.

P.S. The form changes depending on the registration option enabled.

Related #23637