Control codes for
teletext are
precisely the
BBC Microcomputer's
MODE 7 control codes (it was
designed with the
teletext adaptor in mind). Every control code requires a
cell, so if you want to switch e.g. to
blinking
yellow text, 2 blank spaces will show up.
Codes include double height (but you had to start on an even row!), foreground and background colours, and even graphics. In graphics, each character cell is divided into 6 boxes (3 rows, 2 columns); total resolution is 80x75, but generally you mix text and graphics. Each box can be controlled separately, but they must all have the same colour! There's also provision for having the boxes noncontiguous, so you get your pixels with a background-coloured boundary around them.
Designing good teletext pages is something of an art. The BBC even had games written for this mode -- mostly clones of old Atari 2600 games (Space Invaders, Snake, and the like). Storing an entire page of teletext mode requires just 40x25 = 1000 bytes, or under 1K. If you consider that it's hard to read 80 column text on a TV set, you begin to understand the engineering appeal of teletext.
The real killer of teletext wasn't the technology; it was the content. Most broadcasters just repeated their news headlines (and stories) on the teletext pages; why waste your time reading the headlines from your TV set, when you can get somebody with excellent diction to read them out to you while you're brushing your teeth?