Add a ::content pseudo-element.

@@ -23,6 +23,7 @@ export const PSEUDO_ELEMENT = Symbol('PSEUDO_ELEMENT')
 export const PSEUDO_ELEMENTS = {
   after: { type: PSEUDO_ELEMENT, name: '::after' },
   before: { type: PSEUDO_ELEMENT, name: '::before' },
+  content: { type: PSEUDO_ELEMENT, name: '::content' },
 }
 
 export const fixAttributes = (attributes, options = { mapClassname: true }) => {
@@ -304,6 +305,8 @@ const compileMatcher = (ast: AST, selector: string): MatcherProducer => {
             return (context, node, parent, i, debug, special) => special === PSEUDO_ELEMENTS.after
           case 'before':
             return (context, node, parent, i, debug, special) => special === PSEUDO_ELEMENTS.before
+          case 'content':
+            return (context, node, parent, i, debug, special) => special === PSEUDO_ELEMENTS.content
           default:
             console.error('pseudo-class', nodeUtil.inspect({ selector, ast }, { depth: null, colors: true }))
             throw new Error(`Unknown pseudo-class: ${ast.name}`)