I’ve been using the alpha for a few months, and I prefer using the new configuration in the app.css file and deleting the js config. I did not change the config.exs, so it still does the cd into the assets directory.

config :tailwind,
  version: "4.0.0-beta.1",
  my_app: [
    args: ~w(
      --input=css/app.css
      --output=../priv/static/assets/app.css
    ),
    cd: Path.expand("../assets", __DIR__)
  ]

/* if you want dark mode via class */
/* default is to use prefers-color-scheme */
@variant dark (&:where(.dark, .dark *));

@variant phx-click-loading (.phx-click-loading&, .phx-click-loading &);
@variant phx-submit-loading (.phx-submit-loading&, .phx-submit-loading &);
@variant phx-change-loading (.phx-change-loading&, .phx-change-loading &);

@plugin "./heroicons.tailwind.plugin.js";

@source "../js/**/*.js";
@source "../../lib/my_app_web.ex";
@source "../../lib/my_app_web/**/*.*ex";

@import "tailwindcss";

And create a heroicons.tailwind.plugin.js if you use heroicons:

const fs = require("fs");
const path = require("path");

module.exports = function ({ matchComponents, theme }) {
  let iconsDir = path.join(process.cwd(), "../deps/heroicons/optimized");
  let values = {};
  let icons = [
    ["", "/24/outline"],
    ["-solid", "/24/solid"],
    ["-mini", "/20/solid"],
    ["-micro", "/16/solid"],
  ];
  icons.forEach(([suffix, dir]) => {
    fs.readdirSync(path.join(iconsDir, dir)).forEach((file) => {
      let name = path.basename(file, ".svg") + suffix;
      values[name] = { name, fullPath: path.join(iconsDir, dir, file) };
    });
  });
  matchComponents(
    {
      hero: ({ name, fullPath }) => {
        let content = fs
          .readFileSync(fullPath)
          .toString()
          .replace(/\r?\n|\r/g, "");
        let size = theme("spacing.6");
        if (name.endsWith("-mini")) {
          size = theme("spacing.5");
        } else if (name.endsWith("-micro")) {
          size = theme("spacing.4");
        }
        return {
          [`--hero-${name}`]: `url('data:image/svg+xml;utf8,${content}')`,
          "-webkit-mask": `var(--hero-${name})`,
          mask: `var(--hero-${name})`,
          "mask-repeat": "no-repeat",
          "background-color": "currentColor",
          "vertical-align": "middle",
          display: "inline-block",
          width: size,
          height: size,
        };
      },
    },
    { values },
  );
};

I think v4 is a fantastic advancement. It uses native CSS layers, which I find solves all problems I have with using selectors for the base style of components, while still allowing the use of utility classes for overrides.

Got it working:

This is a working config:

  version: "4.0.0-beta.1",
  default: [
    args: ~w(
      --input=assets/css/app.css
      --output=./priv/static/assets/app.css
    ),
    cd: Path.expand("../", __DIR__)
  ]

diff:

image893×171 22.2 KB

As you can see, the difference is to not cd into assets. Tailwind 4 does source detection differently, but you can still configure it: Installing Tailwind CSS with Vite - Tailwind CSS, so the old configuration can probably be kept with correct source detection, but I didn’t bother with that.

As a follow-up, here’s a gist with a simple & robust implementation of dark mode toggling. It contains the config to add to app.css, and button and JS components to add to your core_components.ex file.