I learned what I know of (X)HTML, CSS, PHP, et al., on my own – through trial and error, through a few good books, through online tutorials. One of the side effects of learning this way is a lack of an awareness of institutionally-approved standards.
There is no proper way to organize a CSS stylesheet, I suppose, but I’m sure that if I had learned CSS in school I would have (by choice or by requirement) adopted a professor’s or department’s standard. Instead, I made up my own, and then joined the corporate world and discovered, well, mine’s not the norm.
I like to organize my stylesheets so that all of the declarations for one selector are on one line. I find it easier to scan and, thereby, easier to update and maintain.