Role of lineGap parameter

[andriytyurnikov]

As I am developing CSS-only (PostCSS, strictly speaking) port of Capsize, I went with simple case first, and assumed lineGap:0.
Not only many fonts have lineGap:0, but also few that don't (like Helvetica Neue) don't seem to be affected if I use lineGap:0.
Is it really necessary, or is it a just a legacy of old implementation, which was not using display: table; trick?

Please tell me what am I missing.

[michaeltaranto]

Hey @andriytyurnikov 👋 curious to understand more about what you are looking to achieve, as Capsize is "css-only" (unless you are calling it within your runtime code?).

The lineGap parameter gives consumers a different mental model for defining line-height, describing the desired "gap" between capital letters and baselines. It pairs really nicely with capHeight as you are no longer defining font-size and line-height explicitly, rather the height of your capital letters and the space "between lines".

This works really well in a systems space, where you can better understand the relationship between your space scale and your typography spacing.

[andriytyurnikov]

Oh, I see. Hmm. I failed to grasp it from the start, and sometimes used padding.
Basically, I set lineGap = 0, which allows shortest branch of your formula.
Line-height setting still works on multiline text, and for single line text I added padding sometimes.

Hmm. I'll need some time to digest this.
Thanks!

[michaeltaranto]

Just to be clear, we are talking about the lineGap parameter to the Capsize API right?

There is also the line gap font metric internally, which is populated for some fonts, which I believe is additional line spacing beyond line height.

[andriytyurnikov]

Something, something… ubiquitous language 😂

Yes - absence of connection between those two things with same name is what I struggled with.
Thanks for clarifying.

I guess traditional typography had “leading” for that, but, sure, digital tools , including Tailwind, have butchered traditional terminology long time ago.