Common mistakes for CSS-designers to avoid? [closed]

Question

What are some of the most common mistakes made by CSS-Designers?

Answer 1

Not realizing till a year into it (like me), that you can apply more than 1 class at a time.

.Center {text-align:center}
.Disco {background: red; text-decoration: blink;}
.Highlight { font-weight: bolder;}

    <div class="Disco Center Highlight"></div>

Is valid and it will combine them all.