let ignorePreload = { script: true, image: true }
  function htmlPlugin(tree) {
    tree.walk(i => {
      if (i.tag === 'link' && i.attrs.rel === 'stylesheet') {
        return [{ tag: 'style', content: [css] }]
      } else if (i.tag === 'link' && ignorePreload[i.attrs.as]) {
        return []
      } else if (i.tag === 'script') {
        return {
          tag: 'script',
          content: js
        }
      } else if (i.attrs && i.attrs.class) {
        return {
          tag: i.tag,
          content: i.content,
          attrs: {
            ...i.attrs,
            class: i.attrs.class
              .split(' ')
              .map(kls => {
                if (!classes[kls]) {
                  process.stderr.write(`Unused class .${kls}\n`)
                  process.exit(1)
                }
                return classes[kls]
              })
              .join(' ')
          }
        }
      } else {
        return i
      }
    })
  }