Formatting in SRT is unofficial. SubRip stored plain text; the tags below came from players and editors, and each program supports its own subset. The one reliable rule is that text a reader does not recognise as a tag is shown as it stands.
Alignment codes
Files converted from SubStation Alpha often contain
its numeric alignment codes in braces, such as {\an8} at the start of a
line to put it at the top of the picture. \an takes the position on a
numeric keypad: 1, 2 and 3 are bottom left, centre and right, 4, 5 and 6 the middle
row, and 7, 8 and 9 the top row.
FFmpeg keeps {\anN} and strips other override blocks; it also strips
MicroDVD codes such as
{Y:i}. Support elsewhere varies, and a player without it shows the braces
as text.
Worked example
7
00:00:40,000 --> 00:00:42,500
{\an8}<font color="#FFFF00"><i>[door slams]</i></font>Not confirmed
No source settles how HTML character references such as & should
be treated in SRT. The set of colour names a player accepts also differs between
programs.