PostCSS

PostCSS is a framework for CSS postprocessors. You gives custom JS function to modify CSS and PostCSS parses CSS, gives your usable JS API to edit CSS nodes tree and then save modified nodes tree to new CSS.

For example, lets fix forgotten content ptopery in ::before and ::after:

var postcss = require('postcss');

var postprocessor = postcss(function (css) {
    css.eachRule(function (rule) {
        if ( rule.selector.match(/::(before|after)/) ) {

            var good = rule.decls.some(function (i) {
                return i.prop == 'content';
            });
            if ( !good ) {
                rule.prepend({ prop: 'content', value: '""' });
            }

        }
    });
});

And then CSS with forgotten content:

a::before {
    width: 10px;
    height: 10px;
    background: black
}

will be fixed by our new postprocessor:

var fixed = postprocessor.process(css);

to:

a::before {
    content: "";
    width: 10px;
    height: 10px;
    background: black
}

Sponsored by Evil Martians.