SnapCSS is a tool that extracts all related CSS rules for the given HTML:

const snapCSS = require('snap-css')
const css = '.red { color: red } .green { color: green } .blue { color: blue }'
const html = '<div class='green'>OK</div>'
console.log(snapCSS(css, html))
//=> '.green { color: green }'

It also provides functionality to scrap loaded CSS from real DOM:

const {snapCSSFromDOMOnLoad} = require('snap-css/dom')
snapCSSFromDOMOnLoad(window)
  .then(console.log)
  //=> '.red { color: red } .green { color: green } .blue { color: blue }'

See how it works.

npm install snap-css --save

or

yarn add snap-css

SnapCSS uses PostCSS to process the given CSS code. It walks through every rule and check the query (e.g. div .class or :hover span) against HTML loaded into a JSDOM instance using following algorithm:

• Replace pseudo-classes and pseudo-selectors with empty string before perforimg a query, so that div:hover span turns into div span, :active into *, etc.
• Perform every selector and remove the rules that don't match the DOM.
• Keep all at-rules (e.g. @page, @media, etc)
• Remove unused @keyframe animations and empty rules i.e. empty @media.
• Format CSS using postcss-prettify.

Since the HTML is static, SnapCSS caches every query performed against it, so it allows process repeating queries in the most performant way possible.

It's possible to get false-positive rules, like:

const snapCSS = require('snap-css')
const css = 'div:empty { color: red }'
const html = '<div>Not empty!</div>'
console.log(snapCSS(css, html))
//=> 'div:empty { color: red }'

The problem here is that JSDOM doesn't perfectly match the spec, so some selectors i.e. :optional doesn't work as intended, so that it could lead to false-negative captures which are worse than false-positive because otherwise, means missing styles.

If a selector causes an exception (due to an incorrect or unsupported syntax), the rule will be ignored.

Please report an issue if you found a false-negative scenario, so we can improve the library.

MIT © Sasha Koss