snecss.html

<script type=module>
  import computedVariables from 'https://unpkg.com/computed-variables/index.es.js';

  computedVariables(
    '--python',
    (value, event, rule) => {
      if ('brython' in window === false) {
        const tag = document.createElement('script');

        tag.onload = callback;
        tag.src = 'https://cdnjs.cloudflare.com/ajax/libs/brython/3.8.8/brython.js';
        document.head.append(tag);
      } else {
        callback();
      }

      function callback() {
        const python = document.createElement('script');

        python.type = 'text/python';
        python.textContent = rule.style.getPropertyValue('--python-value').replace(/\\;/g, ';\n');
        document.head.append(python);
        brython();
      }
    },
    window,
    ['load']
  )
</script>

<style>
  #demo {
    --python:
      from browser import document, alert\;

      def hello(ev): alert("hello from python in CSS!")\;

      document["demo"].bind("click", hello)\;
    ;
  }
</style>

<button id=demo>Click me</button>