HTTP Status Codes is a quick reference for every standard code, from 100 to 511, each with a short plain-English meaning and a colour by category. Search by a number, a prefix like 50, or a word from the description.
It is a static reference; nothing is uploaded.
All the standard IANA-registered codes, plus a few common WebDAV ones and 418.
They mark the category by the first digit: 1xx, 2xx, 3xx, 4xx, 5xx.
Yes. A non-numeric query matches words in the name and the meaning.
No. After the first visit it works offline.