Apparently it's not only replaced elements that can have intrinsic
sizes, so let's move this concept from ReplacedBox to Box. To avoid
bloating Box, we make the accessors virtual.
Our "frame" concept very closely matches what the web specs call a
"browsing context", so let's rename it to that. :^)
The "main frame" becomes the "top-level browsing context",
and "sub-frames" are now "nested browsing contexts".
SPDX License Identifiers are a more compact / standardized
way of representing file license information.
See: https://spdx.dev/resources/use/#identifiers
This was done with the `ambr` search and replace tool.
ambr --no-parent-ignore --key-from-file --rep-from-file key.txt rep.txt *
This is because it includes the initial node that the function was
called on, which makes it "inclusive" as according to the spec.
This is important as there are non-inclusive variants, particularly
used in the node mutation algorithms.
For example:
<div>
<input type=radio name=group value=item1 />
</div>
<div>
<input type=radio name=group value=item2 />
</div>
Is a valid DOM and clicking on of these radio buttons should uncheck
the other.
