I am reading Mobile HTML5 written by Estelle Weyle. It is an awesome recap and new knowledge about html, css and javascript. I want to highlight one of tips I got: Use the weakest CSS selectors (can be found on page 204).

We all know, that inline CSS and !important are evil. They have a higher level of specifity and override the standard cascade of the CSS rules. If you use !important, then it will be hard to override CSS rules when you really need it. After these two classic “evils” the evil number three is overqualifying of CSS selectors. You should really never add more classes or ids or elements than needed. Consider this:

It is often enough, you don’t need those:

SharePoint

In SharePoint sometimes we want override some OOB CSS rules. We have our own “namespace” to do that really simple. We have added a class to the html tag: “contoso-html”. Then it was really easy to update any OOB CSS rules, just copy the selector and add .contoso-html:

We also use less preprocessor and nest our selectors which has resulted that almost all of our selectors contain .contoso-html. Now back to the tip of that blog. This “namespace” made it harder to adjust CSS rules for special cases, like left navigation in the Search Center, we were forced to add more specific selectors, and selectors became bigger and bigger.

What I would recommend today is to skip the .contoso-html namespace in the CSS but keep it in the masterpage markup, use it wisely, meaning only if the original rule doesn’t work. Don’t nest too much in less files. Try always to find the weakest CSS selector. It is especially important in the SharePoint “markup djungle”.