A flexible CSS reprocessor using <style> tags

reproCSS is a CSS reprocessor that adds a process="" attribute to the <style> tag in HTML to allow authors to target JavaScript events when reprocessing CSS, as well as interpolate JavaScript.

Can you imagine if you could interpolate JS inside CSS with the ${} syntax, and also control when and how frequently that CSS reprocessed with a process="" attribute on the <style> tag:

<style process="none"></style>
<style process="once"></style>
<style process="auto"></style>
<style process="touchstart mousedown"></style>

If you are using reproCSS with custom events, you may also optionally use a selector attribute specify a list of one or more CSS selectors you would like to add event listeners for. If no selector attribute is found all custom events will be applied to window.

<style process="click" selector="#any, .css, [selector]"></style>

You can add the CSS you want reprocss.js to apply to your HTML in <style> tags with the following values on the process attribute:

How to use reproCSS


Include the reprocss.js JavaScript plugin in your HTML:

<script src="reprocss.js"></script>


If you are using reproCSS on npm you can include it in your JS modules with a line like this:

const reprocss = require('reprocss')

How to write reproCSSed CSS

To evaluate JavaScript inside the CSS as it's being reprocessed by reprocss.js you can use the ${} interpolation syntax. The following <style> tag would always ensure the <div> in this example was half of the window’s height:

<style process="auto">
  div {
    height: calc(${innerHeight}px / 2);

When the browser is 1000px tall the ${innerHeight} in our CSS will be output as 500, leading to the following output:

<style process="auto">
  div {
    height: calc(500px / 2);

Currently this plugin only supports <style> tags, but it may be possible to support CSS loaded via <link> with a similar technique.


Interpolating JS-supplied values in CSS content:;


  var user = 'Username'

<style process="once">
  div:after {
    content: ' ${user}';

Element Queries via a JS Selector Resolver

<div id="demo">

<style process="resize">
  ${demo.offsetWidth > 400 && "#demo"} {
    background: lime;
  ${demo.offsetWidth > 400 && "#demo"} p {
    color: red;

JS interpolation in CSS

<textarea id="demo"></textarea>

<style process="input">
  #demo {
    background: hsl(${demo.value.length}, 50%, 50%)



Writing mixins for reproCSS is easy, any JavaScript function that outputs code that can be used in CSS can be called from anywhere in the stylesheet reproCSS is processing using JS interpolation with ${}.

An example of a common mixin template might look like this:

function mixin(selector, rule) {

  // Find tags in document matching selector
  var tag = document.querySelectorAll(selector)

  // Begin with an empty style
  var style = ''

  // Begin counting matching tags at 0
  var count = 0

  // For each tag matching our selector
  for (var i=0; i<tag.length; i++) {

    // Create an identifier based on the selector used
    var attr = selector.replace(/\W+/g, '')

    // Mark tag with a custom attribute containing identifier and count
    tag[i].setAttribute('data-mixin-' + attr, count)

    // Add a copy of the CSS rule to the style using a selector for this tag's unique attribute
    style += '\n[data-mixin-' + attr + '="' + count + '"] {\n'
             + '  ' + rule + '\n'
             + '}\n'

    // Increment the tag counter by +1


  // Return all generated styles as CSS text
  return style


If you were going to create a mixin starting from the template above the first thing you'd want to do is change the function name (currently mixin()) to something unique, as well as update the mentions of mixin inside the mixin logic where it’s used to name the elements the mixin is styling, data-mixin. Once you have changed the name of the function, you can pass a CSS selector or a list of CSS selectors into to the plugin, along with CSS properties and values as a string to be processed and added to new rules. This basic template can be extended in many ways to support different things. Here are some examples of reproCSS mixins and helper functions:


Techniques Capable