problematic line-height values

[danielgamage]

window.getComputedStyle doesn't always return a pixel value, particularly when it evaluates to normal.

style value	getComputedStyle value
	18px
inherit	18px
initial	normal
normal	normal
revert	18px
unset	18px

mechanical-ragger fails for this test case (since normal evaluates to NaN).

MDN suggests that the default is ~1.2, but chrome uses something closer to 1.16 🥴

normal

Depends on the user agent. Desktop browsers (including Firefox) use a default value of roughly 1.2, depending on the element's font-family.

— https://developer.mozilla.org/en-US/docs/Web/CSS/line-height#values

Either way, relying on all UAs to use a reliable default here feels risky, so maybe we should log a warning in the console and not try to render the exclusion if we can't figure out the line-height? and probably update the README to note this.