Parsing error on content: "\<"; in CSS

[coaxial]

IMPORTANT: clean-css is now in a maintenance mode. PRs are still welcome, and I will try do an occasional bugfix relase.

Precheck

• Do a quick search and make sure a bug has not yet been reported;
• do a quick check if the bug still exists in the latest patch version;
• finally, be nice and have fun!

Environment

• clean-css version - npm ls clean-css: clean-css@5.3.2
• node.js version - node -v: 16.20.0
• operating system: Mint 20.3

Configuration options

Used through html-minifier-terser

Input CSS

.fa-less-than::before {
  content: "\<";
}

Actual output CSS

No output, but an error:

Minification error on _site/assets/css/hello.css
Parse Error: <";
}

Expected output CSS

.fa-less-than::before{content:"\<";}  // I think?