SOURCE

<p id="firstname">My name is Donald.</p>
<p id="hometown" class="class1">I live in Duckburg.</p>
<p id="firstname" class="class2">My best friend is Mickey.</p>
<p id="firstname" class="class2" style="color">My best friend is Mickey.</p>
<p id="hometown" class="class1 class2">I live in Duckburg.</p>
<p id="hometown" class="class2 class1">I live in Duckburg.</p>
<p id="hometown" class="class2" style="color">I live in Duckburg.</p>
<p id="hometown" class="class1" style="color">I live in Duckburg.</p>
<section id="s1">section1</section>
<section id="s2">section2</section>

#firstname
{
background-color:yellow;
}
.class1
{ 
  background-color:#d0e4fe;
}
.class2
{ 
  color:orange;
    text-align:center;
}
#s2
{
  margin:20px;
  Border: 5px solid green;
  Padding: 20px;
  width: 100px;
   height:20px; 
  background-color:#d0e4fe;
}