If, Or & Else custom elements

name attribute checks if FormData includes an entry with that name.
value attribute checks if FormData includes that value
name & value check if FormData includes that value for that name.
Reacts to input & change events. Send an event manually if you need to re-evaluate the ifs in response to something other than user input.
If you need more complex conditions, run your condition expression in javascript and set the expression result as a value on a hidden input, like in expressiontest if clause on this page.

Checkboxes

Some selected Nothing selected
Uno checked, Duo not checked

Check Uno Check Duo

Numbers, expression

First number 10 20

Second number 30 40

First + second = 40

First is 10, second is 30

Number

* 3 = 30

Exists at all

Non empty value

Misc

This shoud always show

Ornum
Ornum is 1, 2 or 3