Prefix class name with js to manipulate with javascript

Have you ever seen a class name starting with `js-`?
If the answer is not, then you can visit the GitHub repository and see the source of generated HTML. There are bunch of elements whose class are prefixed with `js-` such as:
<meta class="js-ga-set" name="userId" content="..." /> <meta class="js-ga-set" name="dimension1" content="..." />
They are often used to manage the list of elements that we don't want to set id for (dynamic elements, for example). In these cases, we use the `document.querySelectorAll('js-xxx')` method to retrieve the elements list.
In the other words, the `js-` classes don't define the styles, but for managing the elements in JavaScript instead. That's why they are prefixed with `js-`.
Follow this naming convention is helpful if you're working in a team. Other engineers will be aware of the class name, and don't remove them when refactoring the page.