How to set up in AntiX Linux (most popular distro for old computers):

1. Menu -> Control Centre -> Desktop tab -> Edit IceWM Settings -> startup

2. Add this line: setxkbmap -option compose:menu

Key sequences (Monniaux)

Hungarian

Result	Press 'compose' and
acute ó	' o
double acute ő	= o
diaeresis ö	" o

General

Result	Press 'compose' and
em dash —	- - -
arrow →	- >
identical ≡	= _
guillemot «	< <
circled ⑫	( 1 2 )
section §	s o
ellipsis …	. .
non-breaking
heart ♥	< 3
eighth note ♪	# e
smile ☺	: )
frown ☹	: (
multiply ×	x x
divide ÷	: -
super ²	^ 2

For subscript, only numbers seem supported (→ html <sub>i</sub>).