Given HTML and CSS’ inherently interconnected nature, it would be remiss of me to not cover some syntax and formatting guidelines for markup.

Always quote attributes, even if they would work without. This reduces the chance of accidents, and is a more familiar format to the majority of developers. For all this would work (and is valid):

<div class=box>

…this format is preferred:

<div class="box">

The quotes are not required here, but err on the safe side and include them.

When writing multiple values in a class attribute, separate them with two spaces, thus:

<div class="foo  bar">

When multiple classes are related to each other, consider grouping them in square brackets ([ and ]), like so:

<div class="[ box  box--highlight ]  [ bio  bio--long ]">

This is not a firm recommendation, and is something I am still testing this myself, but it does carry a number of benefits.

As with our rulesets, it is possible to use meaningful whitespace in your HTML. You can denote thematic breaks in content with five (5) empty lines, for example:

<header class="page-head">
    ...
</header>





<main class="page-content">
    ...
</main>





<footer class="page-foot">
    ...
</footer>

Separate independent but loosely related snippets of markup with a single empty line, for example:

<ul class="primary-nav">

    <li class="primary-nav__item">
        <a href="/" class="primary-nav__link">Home</a>
    </li>

    <li class="primary-nav__item  primary-nav__trigger">
        <a href="/about" class="primary-nav__link">About</a>

        <ul class="primary-nav__sub-nav">
            <li><a href="/about/products">Products</a></li>
            <li><a href="/about/company">Company</a></li>
        </ul>

    </li>

    <li class="primary-nav__item">
        <a href="/contact" class="primary-nav__link">Contact</a>
    </li>

</ul>

This allows developers to spot separate parts of the DOM at a glance, and also allows certain text editors—like Vim, for example—to manipulate empty-line-delimited blocks of markup.

 

reference: cssguidelines