Find which selectors are used in {pages}
@param  {Array}    page          List of jsdom pages
@param  {Object}   css           The postcss.Root node
@return {Promise}
function getUsedSelectors(page, css) {
    let usedSelectors = [];
    css.walkRules((rule) => {
        usedSelectors = _.concat(usedSelectors, rule.selectors.map(dePseudify));
    });
    return jsdom.findAll(page, usedSelectors);
}