Stroke-width not working

[jcward]

Here's a fairly simple graphic:

<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 100 100">
  <g fill="#F00" stroke="#3b5998" stroke-width="4">
    <path d="M28,6h44v16l-22,21l-22-21z" fill="#6d84b4"/>
    <path d="M28,95h44v-16l-22-21l-22,21z" fill="#6d84b4"/>
    <path d="M6,30v42h15l21-21l-21-21z" fill="#afbfde"/>
    <path d="M95,30v42h-15l-21-21l21-21z" fill="#afbfde"/>
  </g>
</svg>

It's supposed to have a stroke:

This library renders it without a stroke width: