Indeed. I'm always ashamed of using it, but I have a stupid tendency to forget css selectors from time to time (I don't do web design or tweaking that often) and the same goes for how string.match or regexp.test or the likes in JS.
Never be ashamed of looking up a reference if it saves you time, besides web standards are too much of a moving target to bother committing all of them to memory.
I suggest memorizing it. Knowing it from the top of your head is much faster than googling or looking it up. That being said, a reference is helpful in the beginning stage.
For me it's not only web development, is most of the CS stuff I need when coding.
Probably it is due to my training as a mathematician, but I usually just keep an index of what I know and what it's used for, but I don't remember the specific details. I.e. Implicit Function Theorem can be used for proofs involving X, Y and Z under hypotheses H, unless you are fancy and go for H2 and more work. I just know what I need and where I can look it up – Nirenberg for fancy, any basic Differential Analysis text for the others, that Russian book on my shelf for a neat simple proof, Moser's for a simple proof and application, the list goes on with 6 or more 7 items. Well, this is actually stretching it a little thin, I could prove the basic differential versions without much trouble, and some functional versions given enough hypotheses.
When coding I'm more or less the same. I know there's a Javascript function that returns true or false when matching against a regexp, I just don't remember its name. Since it's kind of shelved in my mind as regexp test, I just look for this, and it's hole in one. Once I learn something I may not remember it exactly, but I have a pretty good "indexed memory" in the fact that I know that piece of knowledge exists and I've seen it, so I only need a few moments to dig it up.
Of course, I'd rather remember all CSS selectors and rules, or most of the Javascript machinery and the Go standard library. But I'm not doing this every day, so I just use it, every time I do a little more sticks to memory and the rest is easily found. If it got me more than 1 minute to look something up, I'd stick a big post-it in front of me until it sank, so far I have not needed it.
I just can't memorise everything I use so sparingly. If I did I'd still remember useless VBscript (or VB for Applications,) or Pascal. When I'm in X mode (be it Go, Python, Javascript or CSS) I only need to look up things in the first few hours, after a while it's in working memory for a few days. Then it slowly fades, although I usually remember pretty well the general way to do X or at least, what to look for for what I'm interested in.
