CSS Diff Checker
Paste the before and after of a stylesheet and get a unified diff: removed lines in red, added lines in green, unchanged context between. Perfect for reviewing what a refactor really changed.
Settings
The CSS you need to build this yourself
Understand the output, not just copy it — these lessons on our learning path teach exactly what this tool automates:
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How it works
Under the hood is the classic longest-common-subsequence algorithm — the same family Git uses. It finds the largest set of lines the two versions share, and everything outside that set becomes a removal or an addition. That's why moving a line reads as one − and one +.
Diffing works best line-by-line, which rewards well-formatted CSS — one declaration per line. If either side is minified, run it through the Beautifier first and the diff becomes meaningful.
Run the code — try it yourself
Below are v1 and v2 of a button as comments. Read the intended change, apply it to .btn — then paste both versions into the diff above and confirm only your intended lines changed.
Frequently asked questions
Why does one edited line show as remove + add?
Line diffs treat any changed line as a new one; the −/+ pair together shows the edit.
Is there a size limit?
In-browser LCS is quadratic, so the tool caps very large inputs. Diff the file section you care about.
Can it diff HTML or JS too?
It's line-based, so technically yes — but its examples and workflow are tuned for stylesheets.
Keep going
Reviewing your own diffs before committing is the cheapest code-review there is. Keep both versions readable with the Beautifier and consistent with the CSS Property Sorter.
CSSmatic is a non-profit project, made by developers for developers.