Passing Custom CSS To Polymer Element

I would like to be able to pass CSS width to my custom element's shadow DOM. The custom element, called my-list, is defined as below:

Solution 1:

CSS variable names need to start with --

<dom-module id="my-list">
  <template>
    <style>
      #container {
        width: var(--my-list-width, 100%);
      }
    </style>

    <div id="container">
      <content></content>
    </div>
  </template>

  <script>
    Polymer({
      is: 'my-list'
    });
  </script>
</dom-module>

Hint: the <style> tag should be within the <template> tag (this was explained differently a while back in the docs but changed later).

---

If you use Polymer features you need to add is="custom-style" for Polymer to know it needs to process (polyfill) the styles inside this style tag.

<style is="custom-style" type="text/css">
  /* .medium { I think this should work but I fear it doesn't - haven't investigated yet*/
  :root { /*this works */
    --my-list-width: 500px;
  }
</style>

<my-list class="medium">
  <list-entry>1</list-entry>
  <list-entry>2</list-entry>
</my-list>

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