• Be assumption that you have HTML as example and the question here `How to target to HTML element by using pseudo class`?

<div class="wpb_wrapper">
    <div>...</div>
    <p>...</p>
</div>

• With CSS3, you can follow here:

.wpb_wrapper p:first-of-type { ... }
or
.wpb_wrapper p:nth-of-type(1) { ... }

Keyword: CSS selectors, HTML element, pseudo-class, targeting element.