Why should we not start I'd or class name with a number?

Answer 1

maybe #0 and .0 and all numbers are reserved for something else

edit: tried to run with "first" and .first changed to "1" and .1 but the css doesn't stylised the paragraph

Answer 2

maybe you can start with a "_" looks like this:
#_0 or ._0
dont know if it will work