Magic Symbols is a Chrome browser extension that replaces codes such as {T} with corresponding Magic: The Gathering symbols, such as 
.
The symbols are taken from scryfall.com's Manamoji icons, distributed under Public Domain. The file processImages.js is a Node script that reads each symbol, does some processing, and creates a big mapping of symbol codes to image filenames, and stores it in declarative-content/symbols-list.js for use in the extension.
Run processImages.js in Node once.
There's no way to disable replacement per-symbol or per-site.
The script is run only on page load, so any content added to the document after that (EG in a chat) will not be processed.
Only symbols provided by Scryfall are used. Absences include the Planeswalker symbol {PW} (for the Planechase variant); Roman numerals {r1}, {r2}, and so on (for Saga cards); and loyalty symbols (for planeswalker cards).
The silver-bordered Magic symbols are not fully supported. To get long symbols to display correctly, multiple codes are required: for example, for the hundred-mana symbol 
, the sequence of codes is {100-1}{100-2}.