<p class="important urgent warning ">
This paragraph is a very important and urgent warning.</p>

.important.warning {background:silver;}
.important.urgent.warning {background:silver;}

#intro {font-weight:bold;}
<p id="intro ">This is a paragraph of introduction.</p>

p[class="important warning "] {color: red;}
//p[class="important "] {color: red;} error
<p class="important warning ">This paragraph is a very important warning.</p>

p[class~ ="important "] {color: red;}
<p class="important warning ">This paragraph is a very important warning.</p>

a[href* ="w3school.com.cn"]
<a href="http://www.w3school.com.cn/css/">CSS</a>

*[lang| ="en"] {color: red;}
<p lang="en ">Hello!</p>
<p lang="en -us">Greetings!</p>
<p lang="en -au">G'day!</p>
<p lang="fr">Bonjour!</p>
<p lang="cy-en">Jrooana!</p>

ul em {color:red; font-weight:bold;}
<ul>
<li>List item 1
<ol>
<li>List item 1-1</li>
<li>List item 1-2</li>
<li>List item 1-3
<ol>
<li>List item 1-3-1</li>
<li>List item <em>1-3-2</em>
</li>
<li>List item 1-3-3</li>
</ol>
</li>
<li>List item 1-4</li>
</ol>
</li>
<li>List item 2</li>
<li>List item 3</li>
</ul>

h1 > strong {color:red;}
<h1>This is <strong>very</strong><strong>very2</strong> important.</h1>
<h1>This is <em>really <strong>very</strong></em> important.</h1>

h1 + p {margin-top:50px;}
<h1>This is a heading.</h1>
<p>This is paragraph. </p>
<p>This is paragraph.</p>