Cascading Style Sheets are meant to be read by humans as well as browsers, but real stylesheets rarely stay readable. Code copied from a minified file arrives as a single dense line. A stylesheet edited in a hurry accumulates inconsistent spacing, missing semicolons and indentation that no longer matches the nesting. Editing such a file is risky, because it is hard to see which rule you are changing and where one rule ends and the next begins. The CSS Formatter and Beautifier takes messy CSS and returns clean, indented CSS, so you can read the file before you change it.
Formatting is not the same as fixing. A formatter changes whitespace, line breaks and spacing; it does not change which styles apply to which elements. That distinction is the whole value of the tool. After formatting, a{color:red;margin:0} becomes a block you can scan at a glance, with the selector on its own line, each declaration on its own line, a space after each colon, and a closing brace that lines up with the selector. The meaning is identical, but the file is now something a human can work with. A formatter that silently changed values or reordered rules would be dangerous, because you could no longer trust that the formatted file behaves like the original.
How the tool works
You paste CSS, and the tool returns it with consistent indentation, one declaration per line, normalised spacing around colons, and a semicolon before each closing brace where one was missing. Comments are kept, because a comment often explains why a strange-looking rule exists, and deleting it during formatting would destroy information. Nested rules, such as a rule inside a media query, are indented one level deeper than the rule that contains them, which makes the nesting visible in the shape of the text. Everything runs in your browser, so your stylesheet never leaves your device.
The indentation rule is simple but powerful. Each level of nesting adds one step of indentation, so the visual shape of the file mirrors its logical structure. A media query block indents its contents, and a rule inside it indents its declarations further still. When the indentation looks wrong, that is often a clue that a brace is missing or misplaced, even though the formatter itself does not repair broken CSS. Reading a formatted file is therefore also a diagnostic step: structure you can see is structure you can check.
A worked example
Take the minified rule a{color:red;margin:0}. The formatter returns it as four lines: a { on the first line, then two spaces followed by color: red; on the second, two spaces followed by margin: 0; on the third, and } on the fourth. Nothing about the styling changed. The selector is still a, the colour is still red, and the margin is still zero. What changed is only the presentation: each declaration now has its own line, each colon is followed by a space, and the closing brace sits at the same level as the selector.
A nested example shows the indentation rule in action. A media query wrapping a rule for small screens contains that rule one level in, and the rule's declarations sit one level further in again. Before formatting, that structure may be invisible, especially if the original file was minified into one line. After formatting, you can see at a glance which rules are inside the media query and which are outside it, and that distinction decides which screens each style affects. If a rule appears at the wrong indentation level after formatting, stop and check the braces before editing anything, because the file's structure may not be what you assumed.
Common mistakes
The most common mistake is assuming formatting fixed a bug. If a style is not applying, formatting will make the file easier to read, but the cause is usually specificity, a typo in a property name, or a rule being overridden elsewhere. Formatting helps you find those causes; it does not remove them. A second mistake is formatting a file and then editing the unformatted copy by accident, which happens more often than anyone admits when two versions of a file are open at once. Work from the formatted version, or replace the original deliberately and keep a backup.
A third mistake is reformatting an entire large stylesheet just to change one rule, then struggling to review the resulting diff, which shows every line as changed even though only one rule differs. For a one-line fix in a shared codebase, it is often better to match the surrounding style by hand and leave the bulk reformat for a separate, reviewed change. A fourth mistake is trusting a formatter to repair broken CSS. A missing closing brace or a malformed declaration will still be broken after formatting, just more legibly broken. Fix the structure first, then format.
Honest limitations
This formatter normalises spacing and indentation and adds a missing final semicolon before a closing brace, but it does not validate your CSS or repair broken syntax. It does not check whether a property name is real, whether a value is valid for that property, or whether your selectors match anything on the page. It does not minify, although the formatted output can be minified by another tool if you need a production file, and it does not sort properties or merge duplicate rules. Those are separate jobs with separate trade-offs, and a formatter that tried to do them all would risk changing behaviour. Use it for readability, and use other checks for correctness.
Closing
A stylesheet you can read is a stylesheet you can maintain. Paste the messy CSS, check that the formatted structure matches what you intended, and only then start editing. The few seconds formatting takes will save you from editing the wrong rule in a wall of compressed text. For neighbouring jobs, the HTML Formatter and Beautifier and JavaScript Formatter and Beautifier do the same work for other code, while How to Improve Content Readability covers the wider principle that readable work is maintainable work.