In the component era, the close relationship between HTML and CSS demands—from a maintainability perspective at least—colocation. Some people still believe in a "separation of concerns"; but, as a pragmatist, I only see a technical boundary. One that I, like many (most?) other React developers, would prefer didn't exist.

The most basic way to colocate HTML and CSS is rooted in the HTML standard itself in the form of the style attribute. Web frameworks universally support the style attribute, and React even provides an additional layer of "syntactic sugar" convenience known as style objects. In short, the style attribute, often referred to as inline styles, "just works". Natively.

Unfortunately, some inherent "feature gaps" have made inline styles unsuitable for most real-world use cases. Something as trivial as a hover effect can't be done with inline styles because they lack pseudo-classes. Responsive design is out of the question because they don't have media or container queries. At some point, inline styles were supposed to do more; but, tragically, the browser vendors never delivered.

Today, the simplicity of inline styles is easily outweighed by their limitations, a situation that has fueled much resentment and pushed developers toward heavy-handed workarounds like CSS-in-JS, atomic CSS, and "zero-runtime" solutions (each one making different tradeoffs but accepting some form of significant complexity nevertheless). CSS madness.

Personally, I've engaged in CSS madness for several years, even creating a few workarounds of my own. But a recent discovery has dramatically changed the situation for me.

CSS Variables are programmable.

The fallback trick

The var() function, used to access the value of a variable, allows you to provide a fallback value to use in case that variable isn't set. For example, var(--error-color, red) represents the value of --error-color, if set, or otherwise the fallback value of red.

The programmability I mentioned is built upon this simple fallback mechanism.

When the variable is set to initial, the fallback value is used instead.

When the variable is set to an empty value, the empty value is used rather than the fallback value. The empty value has no effect on the declaration.

Thus, a pair of variables can be used to toggle between arbitrary fallback values depending on some condition, like a matching selector or at-rule.

Armed with this knowledge, you can do the unthinkable—implement a hover effect within an inline style:

<a href style="color: var(--hover-on, #18659f) var(--hover-off, #003665)">
  Hover me
</a>
<style>
  * {
    --hover-off: initial;
    --hover-on: ;
  }
  :hover {
    --hover-off: ;
    --hover-on: initial;
  }
</style>

Live demo

As you can see, the mechanism is simple, but the syntax leaves much to be desired:

1. It is difficult to parse the value of the color declaration at a glance.

2. Combining multiple "state variables" requires nesting, which is tedious and difficult to read. For example, activating a hover effect only when the element is enabled looks something like this:

color: var(--enabled-on, var(--hover-on, #18659f) var(--hover-off, #003665)) var(
    --enabled-off,
    gray
  );

Moreover, the supporting style sheet, while compact and highly reusable, is still just boilerplate that I'm sure you'd rather not have to maintain.

But don't worry—I can offer a solution that addresses all of these points, helping to make this approach not only simple, but convenient.

Hooks (not the React kind)

When I found the variable fallback trick, I was very excited about its potential as a solution to CSS madness. But it was also clear that the syntax was a big problem. Some kind of abstraction would still be needed; but at least now it could map cleanly to a simple underlying reality, rather than simply hiding complexity.

So, I set about building CSS Hooks, aiming to fill the feature gaps of the style prop to complete its otherwise unrivaled developer experience.

A hook allows you to tap into a CSS feature that is normally inaccessible within inline styles. The canonical first example is the :hover pseudo-class; but the possibilities include the full range of pseudo-classes, media queries, container queries, and even custom selectors.

Usage

Anywhere you find a style object that you'd like to enhance with hooks, you can simply "wrap" it in a call to the css function:

function Button(props: Props) {
  return (
    <button
      {...props}
      style={css({
        border: 0,
        margin: 0,
        padding: "0.75rem 1rem",
        fontFamily: "sans-serif",
        fontSize: "1rem",
        background: "#333",
        color: "#fff",
        borderRadius: "0.5rem",
      })}
    />
  );
}

From there, it's possible to use the hooks via nested style objects. For example, you might want to apply some declarations when the hover or disabled hooks are activated:

function Button(props: Props) {
  return (
    <button
      {...props}
      style={css({
        border: 0,
        margin: 0,
        padding: "0.75rem 1rem",
        fontFamily: "sans-serif",
        fontSize: "1rem",
        background: "#333",
        color: "#fff",
        borderRadius: "0.5rem",
        hover: {
          color: "#666",
        },
        disabled: {
          color: "#999",
          cursor: "not-allowed",
        },
      })}
    />
  );
}

Using the variable fallback trick, the css function flattens the style object, reducing it to a form that is compatible with the style prop.

Generating CSS

As you saw earlier when I demonstrated the variable fallback trick, it is supported by a small static style sheet. The CSS Hooks system takes care of generating the content of that style sheet for you. All you need to worry about is adding it to the document, e.g. via a <style> element in the root component of your app.

Configuration (custom hooks)

CSS Hooks ships with ~30 "recommended" hooks, including the basics like hover, active, focus, and disabled. But you can easily extend or replace them with your own custom hooks.

This flexibility lets you decide the best way to approach concerns like:

• Dark mode. Will you use a prefers-color-scheme media query, a dark class, a combination, or something else?
• Responsive design. Are you ready to start using container queries, or do you want to stick with media queries?
• Breakpoints. Where do you draw the line between desktop and mobile devices?

It also helps you address less-common use cases without resorting to external CSS. For example, in a recent project, I created a hook that is activated when hovering over the element's previous sibling, and another one that is activated when pressing down on the previous sibling.

Here's my configuration:

const [hooks, css] = createHooks({
  ...recommended,

  previousHover: ":hover + &",
  previousActive: ":active + &",

  dark: "@media (prefers-color-scheme: dark)",
  mobile: "@media (max-width: 499px)",
  desktop: "@media (min-width: 500px)",
});

Next steps

If CSS Hooks seems interesting, please go to css-hooks.com for more information, including a complete guide to getting started.

But before you click away, I would like to ask you for two favors:

1. As a brand-new project, CSS Hooks has very limited exposure. Would you please consider adding a star on GitHub to help others find it?
2. If you can offer any feedback or have ideas for additional framework integrations, please start a discussion.

Thanks in advance for your help.

Wrapping up

As HTML templating and component-oriented architecture have gained traction over the years—making traditional CSS architecture mostly redundant—the lack of native browser support for inline style rules created a vacuum that many talented developers have rightly attempted to fill. Each solution has had some merit and been "good enough" under the right conditions. Perhaps that is why we were too comfortable with our workarounds to notice when CSS Variables quietly brought us a native solution.

At the same time, I don't want to suggest that CSS Hooks is a cure for CSS madness, except perhaps in relative terms. This solution is extremely flexible, but it still requires you to make some decisions upfront (which hooks will you need). This solution is entirely focused on React and JSX frameworks, but I would like all web developers to have a simple styling solution regardless of their framework choices. It is a highly effective treatment, but a step short of a cure.

The real cure would be explicit browser support for nested rulesets in inline styles, like we were supposed to have 20 years ago. Rumor has it, this might be on the way. And whenever it lands, I hope React's style prop API will be updated accordingly.

In the meantime, CSS Hooks gets me about 95% of the way there; and, after experiencing the alternatives, I am pretty happy with that (for now). I hope that Hooks will make your life easier, too.