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