src/language-html/preprocess.js/addIsSelfClosing
function addIsSelfClosing(ast /*, options */) {
  return ast.map((node) =>
    Object.assign(node, {
      isSelfClosing:
        !node.children ||
        (node.type === "element" &&
          (node.tagDefinition.isVoid ||
            // self-closing
            node.startSourceSpan === node.endSourceSpan)),
    })
  );
}