Don't override explicit leading-*, tracking-*, or font-{weight} utilities with font-size utility defaults

[adamwathan]

This PR improves how the text-{size} utilities interact with the leading-*, tracking-*, and font-{weight} utilities, ensuring that if the user explicitly uses any of those utilities that those values are not squashed by any defaults baked into the text-{size} utilities.

Prior to this PR, if you wrote something like this:

<div class="text-lg leading-none md:text-2xl">

…the leading-none class would be overridden by the default line-height value baked into the text-2xl utility at the md breakpoint. This has been a point of confusion and frustration for people in the past who are annoyed they have to keep repeating their custom leading-* value like this:

<div class="text-lg leading-none md:text-2xl md:leading-none lg:text-4xl lg:leading-none">

This PR lets you write this HTML instead but get the same behavior as above:

<div class="text-lg leading-none md:text-2xl lg:text-4xl">

It's important to note that this change only applies to line-height values set explicitly with a leading-* utility, and does not apply to the line-height modifier.

In this example, the line-height set by text-sm/6 does not override the default line-height included in the md:text-lg utility:

<div class="text-sm/6 md:text-lg">

That means these two code snippets behave differently:

<div class="text-sm/6 md:text-lg">…</div>
<div class="text-sm leading-6 md:text-lg">…</div>

In the top one, the line-height md:text-lg overrides the line-height set by text-sm/6, but in the bottom one, the explicit leading-6 utility takes precedence.

This PR applies the same improvements to tracking-* and font-{weight} as well, since all font size utilities can also optionally specify default letter-spacing and font-weight values.

We achieve this using new semi-private CSS variables like we do for things like transforms, shadows, etc., which are set by the leading-*, tracking-*, and font-{weight} utilities respectively. The text-{size} utilities always use these values first if they are defined, and the default values become fallbacks for those variables if they aren't present.

We use @property to make sure these variables are reset to initial on a per element basis so that they are never inherited, like with every other variable we define.

This PR does slightly increase the amount of CSS generated, because now utilities like leading-5 look like this:

  .leading-5 {
+   --tw-leading: 1.25rem;
    line-height: 1.25rem;
  }

…and utilites like text-sm include a var(…) lookup that they didn't include before:

  .text-sm {
    font-size: 0.875rem;
-   line-height: var(--font-size-sm--line-height, 1.25rem);
+   line-height: var(--tw-leading, var(--font-size-sm--line-height, 1.25rem));
  }

If this extra CSS doesn't feel worth it for the small improvement in behavior, we may consider just closing this PR and keeping things as they are.

This PR is also a breaking change for anyone who was depending on the old behavior, and expected the line-height baked into the md:text-lg class to take precedence over the explicit leading-6 class:

<div class="text-sm leading-6 md:text-lg">…</div>

Personally I am comfortable with this because of the fact that you can still get the old behavior by preferring a line-height modifier:

<div class="text-sm/6 md:text-lg">…</div>

[thecrypticace]

One small thing I noticed but otherwise looks right to me