class Elements extends Array

Elements.js

import { absolutizeRelativeSelectorList } from "http://dev.w3.org/csswg/selectors/#absolutizing";

// Assume JSIDL conversions have been applied already,
// i.e. we don't do `selectors = String(selectors)` manually.
class Elements extends Array {
    query(selectors) {
        return this.queryAll(selectors)[0]; // highly inefficient obviously, but clear semantics
    }
    
    queryAll(selectors) {
        const absolutized = absolutizeRelativeSelectorList(selectors);
        // TODO: instead of `querySelectorAll` possibly use `[[QuerySelectorAll]]` internal
        // implementation, see
        // http://lists.w3.org/Archives/Public/public-script-coord/2013JulSep/0350.html
        const finder = element => this.constructor.from(element.querySelectorAll(absolutized));

        // `allFound` is now an `Elements` but it contains `Elements` instances; we need to flatten this out.
        // My kingdom for an `Array.prototype.flatMap`!
        const allFound = this.map(finder);
        
        // Note that we are concating `Elements` with other `Elements` so it works fine.
        const flattened = allMatching.reduce((a, b) => a.concat(b));

        return flattened;
    }
}

Elements.jsidl

class Elements extends Array {
    Elements queryAll(ToString selectors?)
    Element query(ToString selectors?)
}

This is lovely