JavaScript
JavaScript is minified with Terser (compression and name mangling, license comments kept). Syntax errors are reported with line and column, and nothing is output for invalid code.
CSS, HTML, JSON and XML
CSS and HTML minification here is deliberately conservative: comments and redundant whitespace go, but spacing inside calc(), strings, pre and textarea content, and selectors where a space changes meaning are left alone. It does not merge rules or rewrite values, which is why it is labelled APPROX. JSON is compacted with JSON.stringify after a strict parse.
Questions
Is the gzip size accurate?
It is computed with a real DEFLATE implementation at level 9, close to what most servers send.
Does it keep license comments?
Yes. Comments starting with /*! or containing @license or @preserve are kept.
Can minification break my CSS?
The CSS minifier only removes whitespace and comments where that is safe; it does not restructure rules.