how to use classname in css code example

Example 1: how to make a class in html

<ul class="legend">
    <li><span class="greendot"></span>Text</li><br></br>
    <li><span class="yellowdot"></span>Text</li><br></br>
    <li><span class="reddot"></span>Text</li><br></br>
    <li><span class="blackdot"></span>Text</li><br></br>
</ul>

Example 2: css class attribute

<h1 class="Class">Content</h1>
<h1 id="id">Content</h1>
Css: 
.class
#id