Test example here https://en.wikisource.org/wiki/User:ShakespeareFan00/Sandbox#Formatting_Side-titles, The aside is not seemingly being parsed. The reason I found this issue, was that i was trying to sandbox and work on a solution to how to do consistent sidetitles in a more consistent manner than the rather cumbersome kludge of template I had been using to this point.
|Open||None||T21719 HTML5 features (tracking)|
|Resolved||None||T12467 Use semantic HTML (tracking)|
|Open||None||T25932 Enable, whitelist, and incorporate semantic HTML5 elements|
|Open||None||T104770 HTML5 Semantic element <aside></aside> not seemingly parsed.|
The HTML5 recommendation says:
The aside element represents a section of a page that consists of content that is tangentially related to the content around the aside element, and which could be considered separate from that content. Such sections are often represented as sidebars in printed typography.
The element can be used for typographical effects like pull quotes or sidebars, for advertising, for groups of nav elements, and for other content that is considered separate from the main content of the page.
Note: It's not appropriate to use the aside element just for parentheticals, since those are part of the main flow of the document.
So it's not really the thing to use for side-titles (not that using elements for other than their original semantic purpose is particular foreign to MediaWiki!) but they're probably the thing to use for sidenotes (c.f. Wikisource:Template:Sidenotes begin).
Regardless, I think it'd be great to at least render them! Skins can make up their mind what to do with them, then.
See the WAI-ARIA 1.1 role definitions to get started.
An element's role corresponds to its semantic meaning — how a human understands the element's place in the document structure. Most of the time, elements won't have a role attribute, because the element's default role is the intended semantics. For example, <li>'s default role is listitem, so <li> used as an item in a list should have no role attribute. The role attribute, if present, overrides the element's default role — <table role="presentation"> is common for layout tables — or provides semantics to a semantically void element, such as <div>.
So I use a <div> with the role attribute to substitute for HTML5 semantic elements. First, I look at the element's default role, then I use that for the <div>'s role attribute. In <aside>'s case, <aside>'s default role is complementary. So, where I would normally use <aside> but also need no-JS IE8 support, I'd instead use <div role="complementary">. Screen readers should announce and navigate <div role="complementary"> identically to <aside>; I don't know any screen readers that don't.
If I were going to use a non-default role on the element, for example <aside role="note"> for a "see also" hatnote, I just use <div> with the same role attribute: <div role="note">.