Previously, tidy-html5 (`tidy`) would complain about a few things in our
HTML. The main thing is that `<summary>` tags can't contain `<div>`s.
That's easily fixed by changing out the `<div>`s for `<span>`s with
`display: block`.
However, there's also a rule that `<span>`s can't contain heading
elements. `<span>` permits only "phrasing content"
https://developer.mozilla.org/en-US/docs/Web/HTML/Element/span, and
`<h3>` (and friends) are "Flow content, heading content, palpable
content".
https://developer.mozilla.org/en-US/docs/Web/HTML/Element/Heading_Elements
We have a wrapping `<div>` that goes around each `<h3>`/`<h4>`,
etc. We turn that into a `<section>` rather than a `<span>` because
`<section>` permits "flow content".
https://developer.mozilla.org/en-US/docs/Web/HTML/Element/section
After this change we get only three warnings from tidy, run on
struct.String.html:
line 6 column 10790 - Warning: trimming empty <span>
line 1 column 1118 - Warning: <link> proprietary attribute "disabled"
line 1 column 1193 - Warning: <link> proprietary attribute "disabled"
The empty `<span>` is a known issue - there's a span in front of the
search box to work around a strange Safari issue.
The `<link>` attributes are the non-default stylesheets. We can probably
refactor theme application to avoid using this proprietary "disabled"
attribute.
