• Style based on context, <em> elements within <h1> elements say
• Take precedence over simple selectors

p em { color: green}
p em strong{ color: brown}
p strong {color:red}
div p em strong{ color:blue}
......
<p>A paragraph with <em>emphasis</em> and <strong>strength</strong> 
 and <em><strong>emphasised strength</strong></em></p>
<div><p>A paragraph with <em>emphasis</em> and <strong>strength</strong> 
 and <em><strong>emphasised strength</strong></em> inside a div</p></div>

Strong emphasis within a paragraph would be coloured blue if the paragraph was part of a div

Effectively, the selector is read backwards: any strong that is part of an em that is a descendant of a p that is a descendant of a div

A paragraph with emphasis and strength and emphasised strength