1. @DomName('DocumentFragment.querySelector')
  2. @DocsEditable()
Element querySelector(String selectors)

Finds the first descendant element of this document fragment that matches the specified group of selectors.

selectors should be a string using CSS selector syntax.

var element1 = fragment.querySelector('.className');
var element2 = fragment.querySelector('#id');

For details about CSS selector syntax, see the CSS selector specification.

Source 

@DomName('DocumentFragment.querySelector')
@DocsEditable()
Element querySelector(String selectors) =>
    _blink.BlinkDocumentFragment.instance
        .querySelector_Callback_1_(this, selectors);