What minifying removes
Browsers ignore almost all the whitespace in a stylesheet. The indentation, the newlines between rules, the space after each colon and every comment you have written exist purely for people. Minifying strips all of it: comments go entirely, runs of whitespace collapse, the spaces around braces, colons, semicolons and commas disappear, and the final semicolon before each closing brace is dropped. What is left is the same stylesheet, byte for byte smaller, and the tool reports exactly how much smaller.
Everything runs in your browser, so stylesheets from private projects are never uploaded anywhere.
Worked example
The first example is typical of hand-written CSS: comments and indentation routinely account for 15 to 40 percent of a file. The second is why careless minification is dangerous. Inside calc(), the plus and minus operators are only valid with a space on each side, so calc(100%-20px) silently fails. This tool never touches spaces around plus or minus signs, and quoted strings, such as content values and font names, are copied through verbatim.
Honest scope: what this tool does not do
This is a whitespace and comment stripper, deliberately conservative so that valid CSS in means valid CSS out. It does not shorten six-digit hex colours to three, rewrite 0.5em as .5em, merge duplicate selectors, reorder declarations or remove rules your pages never use. Build-time minifiers such as cssnano and Lightning CSS do all of that and typically squeeze out a few extra percent, at the cost of a toolchain and the occasional subtle rewrite bug. For pasting a stylesheet in and getting a smaller one out with zero risk and zero installation, conservative is the right trade.
The same conservatism applies to at-rules. Media queries keep the space between and and the opening bracket, because @media screen and (min-width:600px) is only valid with it, and font-face, keyframes and custom property declarations all pass through with just their padding removed. If the output ever looks wrong, the original is still sitting in the input box, untouched.
One habit worth keeping: minify a copy, not your source. The commented, indented file is the one you edit and keep in version control; the minified file is a build product you can regenerate here in seconds whenever the source changes.
Minification and compression are different savings
Your server almost certainly compresses CSS with gzip or Brotli before sending it, and compression already squashes repeated whitespace very efficiently. So does minification still matter? Yes, but less than the raw numbers suggest. A file that minification shrinks by 40 percent might only transfer 10 to 15 percent smaller once compression has done its work, because the two techniques overlap. Minification still wins on parse time, on cache storage, and on the compressed size itself, since fewer bytes in means fewer bytes out. The cheap, sensible default for production is both: minify, then serve compressed.
Frequently asked questions
Is my CSS uploaded anywhere?
No. Minification runs entirely in your browser with JavaScript, so stylesheets never leave your device. You can safely paste CSS from private or unreleased projects.
Will minifying break my CSS?
This tool only removes comments and whitespace that browsers ignore, and it leaves quoted strings and the spaces inside calc() expressions alone, so valid CSS stays valid. It does not rewrite values or shorten colours, which is where aggressive minifiers occasionally introduce bugs.
Why does calc() need its spaces?
Inside calc(), the plus and minus operators must have a space on each side, so calc(100% - 20px) is valid but calc(100%-20px) is not. A careless minifier that strips those spaces silently breaks the declaration. This tool never removes spaces around plus or minus signs, so calc() expressions pass through intact.
How much smaller will my CSS get?
It depends on how it is written. Typical hand-written CSS with normal indentation and a sprinkling of comments shrinks by roughly 15 to 40 percent. Heavily commented files shrink more, and already compact CSS shrinks less. The before and after sizes are shown so you can see the real figure for your file.
Is this a replacement for a build tool?
No. Build-time minifiers such as cssnano and Lightning CSS go further: they merge rules, shorten colours and values, and drop unused prefixes. This tool is for the quick case where you want a smaller stylesheet right now without installing anything. For a production pipeline, use a build tool and serve the result compressed.