CSS minifier

What it removes

Everything a browser does not read: comments, indentation, newlines, the space after every colon and comma, and the final semicolon in each block. On a hand-written stylesheet that is usually 20 to 35 percent of the file.

.card {
  border: 1px solid #dee4ee;
  border-radius: 12px;
  padding: 20px 22px;
}

.card{border:1px solid #dee4ee;border-radius:12px;padding:20px 22px}

It does not rewrite values. Colours stay in the notation you wrote them in, 0.5em keeps its leading zero, and shorthand is not collapsed. Those transformations save more bytes and carry more risk — they belong in a dedicated build tool like cssnano or Lightning CSS, which have test suites behind them.

Where CSS minification actually pays

CSS is render-blocking. The browser will not paint until it has your stylesheet, which makes it worth more attention than HTML byte-for-byte. That said, the ordering of wins is not what people expect:

If your stylesheet is genuinely slowing the page down, unused rules are almost certainly the bigger problem. Most sites ship a framework and use a fraction of it.

Safe by design

The minifier tracks strings, comments and parentheses the same way the formatter does, so:

• A semicolon inside url(data:image/svg+xml;base64,...) does not end the declaration.
• A brace inside content: "}" does not close the block.
• Spaces inside calc(100% - 2rem) are preserved — remove them and the expression breaks.
• Spaces inside grid-template-areas strings are preserved.

Those four are where naive regular-expression minifiers fail, and they fail silently.

Doing it in a build

npx cssnano input.css output.css

module.exports = {
  plugins: {
    autoprefixer: {},
    cssnano: { preset: "default" }
  }
};

Vite, Next.js, Astro and every modern bundler minify CSS in production builds already. Check before you add anything — you may be doing this twice.

This page is for the cases with no build: a single landing page, a snippet going into a CMS field, an email template, or a quick check of how much a file would shrink.

Questions about this tool

Related tools

Learn the why, not just the how

Longer reading on formatting, indentation and minification.