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CSS filters and formatters in golang

Build Status

css cut

Use awesome CSS frameworks without the weight by cutting out unused rules.

css cut is similar to purgecss (GitHub). It scans your HTML for elements, classes and identifiers and then cuts out any CSS rule that doesn't apply. The results for a small site using a framework like bootstrap can be dramatic:

Bootstrap css cut savings
uncompressed 141k 5.6k 96%
compressed 20k 1.8k 91%

See also Hugo #4446


For use with hugo using bootstrap:

# install the binary
go get

# build your site, by default the output is in `public`

# create new minimized CSS file from bootstrap
curl -s | \
    css cut --html 'public/**/*.html' > static/bootstrap-csscut.min.css

Of course, you'll need to use the new bootstrap-csscut.min.css file in your template source.

Be sure to put the HTML file pattern 'public/**/*.html' in single quotes.


TK - likely to change, feedback welcome


TK - likely to change, feedback welcome

How It Works

The Correct Algorithm

The "correct way" to strip out CSS rules might be:

  1. Read in all the CSS files, and extract all the selectors
  2. For each HTML file, execute each selector and see if it returns anything
  3. Use that data to the emit each CSS file with only the selectors that were used.

There are a few problems:

  1. Slow, as you are executing n CSS rules against m HTML files.
  2. Need a perfect CSS Level 3 (or 4!) selector library, else you might strip out rules that are actually used.
  3. Need to know which pseudo-classes matter and which ones do not. For instance :hover can be ignored, but :last_child needs to be evaluated.

The CSSCut Algorithm

Since the Correct Way seems problematic, csscut does the following:

  1. Read each HTML file and keep track of elements, classes and ids found.
  2. Scan the CSS file and convert a selector into a set of "basic selectors". If a rule is h1 h2 h3 then the list of basic selectors is h1, h2, and h3. Classes and identifiers (ids) are preserved, while pseudo elements and attribute selectors are ignored.
  3. Then if each of the basic selectors is found in out list in the first step, the original selector is preserved. This not the rule is tossed out.

As a special case, "universal selectors" are passed through: *, ::before, ::after, ::root. Pure attribute selectors are also passed through: [hidden].

In practice this works well for "flat" CSS frameworks such as bootstrap. For highly specified CSS it might not work as well.

css format

Makes minified CSS readable.

css format < bootstrap.min.css

css minify

css minify < bootstrap.min.css

minify is a shortcut of 'css format' with all options selected to produce the smallest output. It is "conservative" in that it only removes whitespace and does not do any property value rewriting.

css count

See commonly or rarely used CSS classes and identifiers.

Work in progress