Things you look up rather than learn
Every developer has a set of facts they've looked up dozens of times and never retained — the exact difference between a 502 and a 504, the flag that discards local changes, the regex syntax for a non-capturing group. That's not a knowledge gap. Some information is genuinely better looked up than memorized, because it's arbitrary, rarely used, and expensive to get wrong from memory.
These pages are for exactly that: fast lookup, no tutorial preamble.
HTTP status codes
The single most-searched reference in web development, because the codes are numerous and the distinctions are subtle. The classes are worth internalizing even if the specifics aren't: 2xx succeeded, 3xx redirects, 4xx the client did something wrong, 5xx the server did.
The 4xx/5xx boundary is the one that matters when debugging, because it tells you which side to investigate. A 401 means unauthenticated and a 403 means authenticated but not permitted — a distinction that determines whether the fix is logging in or changing permissions.
Regex
Regular expressions are write-often, read-rarely: the syntax is dense enough that most people re-derive it every time. Anchors, character classes, quantifiers, groups, and lookarounds are the pieces, and the cheat sheet covers them with examples rather than formal grammar. Pair it with the regex tester if you're building a pattern rather than reading one.
Vim and git
Both are tools where the common operations are quick and the occasional ones are unmemorable. For vim, that's usually the motions and the specific incantation to quit. For git, it's the recovery commands — undoing a commit, discarding local changes, moving work to the right branch — which are precisely the commands you need under pressure and can least afford to guess at.
Character and encoding tables
ASCII and HTML entities answer the byte-level questions: which character a code point maps to, and how to write a character that would otherwise be interpreted as markup. The time zone table covers abbreviations, which are genuinely ambiguous — several are used for more than one offset, which is why systems that care use IANA identifiers instead.
All static, all local
These are lookup tables rather than tools — they load once and work offline, with no requests and nothing sent anywhere. Fast enough to check mid-task without breaking your train of thought, which is the whole point of a reference.