email.html

<!-- This is how it's done for a normal webpage -->
<style>
.my-paragraph {
 font-size: 12px;
 font-weight: bold;
} 
</style>

<p class="my-paragraph">
 This is my styled paragraph.
</p>
<p class="my-paragraph">
 This is my second styled paragraph
</p>

<!-- This is how it needs to be done for an HTML email -->
<p style="font-size: 12px; font-weight: bold;">
 This is my styled paragraph.
</p>
<p style="font-size: 12px; font-weight: bold;">
 This is my second styled paragraph.
</p>