CSS file size and number of HTTP requests caused
Is the most critical factor in CSS performance
Reflow and rendering time
(Very!) Not that important
Duplication is worse than stale rules
Because we have the tools to handle the latter
Define default values
Don't repeat coding everywhere
The bad:
#weatherModule h3{color:red;}
#tabs h3{color:blue;}
recommend:
h1, .h1{...}
h2, .h2{...}
h3, .h3{...}
h4, .h4{...}
h5, .h5{...}
h6, .h6{...}
Use separate classes to define structures
Don't repeat coding everywhere
Use class
instead of element selector
The bad:
div.error{...}
recommend:
.error{Most of the code is written here}
div.error{separately defined}
p.error{defined separately}
em.error{separately defined}
Source: 99css