Olivero: Adjustments to landmark regions

1. Change the current <main> tag into a <div> : Verified and screenshot attached.
2. Change the content below region's HTML wrapper from aside to <div>: <aside> element changed to <div> : Verified.
3. Change the wrapper around the content region to a <main> : verified.'

Tested on live preview: https://3210199-landmark-regions-bsbshcsepiesqupllmobkmufp0yenlfq.tugboa...

This looks good. Moving the into region--content template makes a lot of sense. Marking as RTBC.

Before this can be committed we need an a11y maintainer signoff. Hence the tag from @mherchel...

This isn't the case on every page, but in some instances such as https://3210199-landmark-regions-bsbshcsepiesqupllmobkmufp0yenlfq.tugboa..., the h1 is not part of any landmark, and it's best practice that all page content be contained by landmarks

It may be possible to wiggle around this by associating the H1 with a main via aria-labelledby, but that would likely be messier than just ensuring the H1 belongs to a landmark region.

If that is the case, it appears there are other regions that won't be in a landmarked element.

{{ page.highlighted }}
{{ page.breadcrumb }} - Has one because it is on thr element. But if anything else is put in this region, it wouldn't be landmarked.
{{ page.content_above }}
{{ page.content_below }}
{{ page.social }}

I suspect based on the desire to have main and sidebar in their own aria regions and not nested, the complementary role needs to be added to the remaining regions.

There's some cool stuff going on in #11, but with this being a stable blocker, it seems reasonable to scope this to target objective accessibility errors. The ones requiring discussion can be in a followup.

Running axe dev tools on the front page of Umami, but with Olivero as the default theme, I get the following errors:

Aside should not be contained in another landmark: <div id="block-olivero-syndicate" class="contextual-region block block-node block-node-syndicate-block" role="complementary">
Landmarks should have a unique role or role/label/title (i.e. accessible name) combination: <nav class="toolbar-lining clearfix" role="navigation"></nav>
Landmarks should have a unique role or role/label/title (i.e. accessible name) combination <aside class="social-bar__inner" data-drupal-selector="social-bar-inner">

You'd likely need to try different pages/configs to confirm there aren't additional problems.

These are clear-cut enough to be obvious stable blockers. As long as you're not adding landmark regions in this issue, fixing these is not an affront to the other landmark related concerns brought up in #3194718: Plan for Olivero theme regions and landmark regions.

Applied the patch #16 looks good to me. Attached screenshot here.

+ RTBC

This passes accessibility review, it now addresses the specific accessibility issues that the issue was created for.

The actual code changes in the patch still need to be reviewed before it can be RTBC'd. (#17 looks like it was just a before/after confirming the design change happened). I'm not able to do that part at the moment, but hopefully the accessibility signoff means another reviewer can focus on the code portion.

If a review results in additional iterations of the patch and any markup structure changes, this should get re-tagged with "needs accessibility review". If it's just css changes or attributes that don't impact accessibility, there's no need.

Verified and tested patch #16.
Patch applied successfully and looks good to me.

Testing Steps:
# Goto: admin/structure/block
# Place "User login" block under sidebar region
# Go to any content
# View the content
# Check the adjustment of user login which is on the right side

Expected Results:
# The sidebar will now top-align with the page title
# Page should be accessible via screenreader with proper focus

Actual Results:
# The sidebar is not top-align with the page title

Note: Tested accessibility and everything is working perfectly. Please check the attached video for the same.

Looks good to me.
Can be a move to RTBC.

Committed 6a53356 and pushed to 9.3.x and cherry-picked to 9.2.x since Olivero is experimental. Thanks!