The computed `font-size` and `line-height` must be clamped when the specified value is out-of-range

[upsuper]

Although it is invalid to specify something like font-size: -1px, it is allowed to do font-size: calc(-1px). In the latter case, font-size must be computed to zero rather than -1px.

The spec: https://drafts.csswg.org/css-values-3/#calc-range

The relevant code is probably https://github.com/servo/servo/blob/master/components/style/properties/longhand/font.mako.rs#L378-L382

(I found font-size and line-height, but other properties may be affected as well.)

[Maaarcocr]

What I'm going to say may be really wrong, due to my ignorance about how style works in servo.

Wouldn't it be better if every property that can implement calc() should provide a maximum and minimum value for calc()?

[Manishearth]

We should add a to_computed_value_positive method that takes the result of to_computed_value() and clamps it.

[upsuper]

That should be to_computed_value_non_negative. I actually think we should somehow put non-negative into the type system, so that we can enforce it everywhere. Many properties would need this.

[nox]

I am pretty sure I fixed this in #16382.

[upsuper]

Actually it isn't fixed. The issue can still be reproduced with

<div id="test"></div>
<script>
test.style.fontSize = 'calc(-1px)';
alert(getComputedStyle(test).fontSize);
</script>

[nox]

servo/components/style/properties/longhand/font.mako.rs

Lines 801 to 805 in 9a419ec

	SpecifiedValue::Length(LengthOrPercentage::Calc(ref calc)) => {
	let calc = calc.to_computed_value(context);
	calc.length() + base_size.resolve(context)
	.scale_by(calc.percentage())
	}

[nox]

I'm not sure to_computed_value_non_negative would make a lot of sense on CalcLengthOrPercentage, we can't know for sure whether 30px - 10% is negative or not. font-size is a special snowflake though.

[nox]

Actually am wrong, but there should still not be any to_computed_value_non_negative method, instead calc() values should remember that they should clamp themselves when needed.