Kind of a general question, but I'm wondering if anyone knows for sure which is best.

When I have a stylesheet where a declaration exists several times, I tend to think I can simplify and clean things up by applying it to multiple classes and Ids at a time. This seems best practice, but I wonder if it's really any faster.

example... which is faster, better, best practice? Imagine a scenario where it isn't just three items, but 20 or more. (note: To simplify, I wrote ''several other declarations" rather than a bunch of random declarations.)

This:

.item1 { width: 100%; /* several other declarations */ }
.item2 { width: 100% /* several other declarations */}
.item3 { width: 100% /* several other declarations */}

Or this, which I tend to do... but I'm not sure it's really any quicker... sometimes it even seems like it's More lines, though I'm trying to get rid of duplication:

.item1,
.item2,
.item3 { width: 100%; }

.item1 { /* several other declarations */ }
.item2 { /* several other declarations */ }
.item3 { /* several other declarations */ }

You mean quicker to load? One of them would probably turn out to be quicker, if we had a way to measure that. But I doubt there would be any noticeable difference, not even on the slowest of servers with a really slow connection.

I'd go for what you find most readable and the easiest to maintain.

That's sometimes a good idea, though it can result in long lists of selectors that become hard to read:



Another thing you can do is give elements two or more classes, like this:

CSS


HTML

But again, you sometimes end up with too many CLASS names for each element, like:


(this seems to be common with blog scripts and similar, maybe it's easier to write the PHP on server-side that way).

Even shorter might be to take advantage of the HTML structure and use CSS child or sibling selectors to find each child element. Then you don't need to create any classes in the HTML, but the CSS can become a bit complex and/or terse, and it's not very portable, so if you ever change the HTML you may have to redo it all.

Personally I like to use HTML container elements, like this:

CSS


HTML

In my experience that strikes a good balance between complexity and CSS+HTML code.

I'd listen to Christian J, you really need to keep in mind to have complexity and CSS/HTML code balanced.