Editorial: Markdown/markup and link default tweaks

[inexorabletash]

• <aside> is a flow element; don't use it like a block. Use <div>
• Use "note:" markdown, which Bikeshed styles better
• Disambiguate "user agent" link

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[inexorabletash]

@saschanaz can can you review? (low priority)

[saschanaz]

Perhaps escape <aside> in the commit message?