[marrs]

Mixing classes and IDs in CSS selectors causes headaches at scale because IDs change the order of precedence. These days I just avoid using them in CSS altogether.

[rfnslyr]

Any articles on this? I've been coding CSS for quite awhile and I didn't know about any of this.

[hhaidar]

http://css-tricks.com/specifics-on-css-specificity/