Do we create "table border=2" without px or % as it is redundant?

Question

I'm confused what the 2 is in reference to. I take it the number is arbitrary, but even if it were a 4 or a 6-- what is it in reference to?

Answer 1

Yes.
2 here is the number of border lines

Answer 2

i think it's pixel, but when you look carefully. there actually 2 lines on the table. except you collapse the border

Answer 3

let's try a there in there and see if there are then 3 lines...

Answer 4

the outside line became very think when I put in eight. there may be a nuance to it but it appears to be in pixels by default