This isn't included in the base definition of a CSS-wide keyword, but
the CASCADE-4 spec which adds it says:
> The revert CSS-wide keyword rolls back the cascade to the cascaded
value of the earlier origin.
So it is one. While I'm at it, rename `is_builtin()` to match the spec
terminology. It's not used currently, but will be in the next commit.
The `to_string()` for this is modified a little from the original,
because we have to calculate what the layer-count is then, instead of
having it already calculated.
This one is a bit fun because it can be `add(<integer>)` or `auto-add`,
but children have to inherit the computed value not the specified one.
We also have to compute it before computing the font-size, because of
`font-size: math` which will be implemented later.
This saves us from having to manually write these every time we add a
new type of StyleValue:
- bool is_foo() const;
- FooStyleValue const& as_foo() const;
- FooStyleValue& as_foo();
This is a universal value like `initial` and `inherit` and works by
reverting the current value to whatever we had at the start of the
current cascade origin.
The implementation is somewhat inefficient as we make a copy of all
current values at the start of each origin. I'm sure we can come up with
a way to make this faster eventually.
Having one StyleValue for `<number>` and `<integer>` is making user code
more complicated than it needs to be. We know based on the property
being parsed, whether it wants a `<number>` or an `<integer>`, so we
can use separate StyleValue types for these.
Specifically, stop letting NumericStyleValues holding `0` from
pretending to hold a Length. The parser is now smart enough that we
don't have to do this. :^)
Only NumericStyleValue holds integers.
I'm not sure our current distinction between NumericStyleValue holding
an integer or non-integer is useful given it always returns a float.
:thonk:
The Display class already supported all specific values, and now they
will be parsed too. The display property now has a special type
DisplayStyleValue.
Rather than passing an increasingly-unwieldy number of font parameters
individually to every function that resolves lengths, let's wrap them
up.
This is frustratingly close to being `Gfx::FontPixelMetrics`, but bitmap
fonts cause issues: We choose the closest font to what the CSS
requests, but that might have a wildly different size than what the
page expects, so we have to fudge the numbers.
No behaviour changes.
This will be needed to access the color of a stop from a SVG gradient
<stop> element (which does not participate in layout, so does not have
a layout node).
Now that LengthStyleValue never contains `auto`, IdentifierStyleValue is
the only type that can hold an identifier. This lets us remove a couple
of virtual methods from StyleValue.
I've kept `has_auto()` and `to_identifier()` for convenience, but they
are now simple non-virtual methods.
