What is difference between writing border="2" and border="2px"...?

Answer 1

Both are equivalent in previous version of Html, but are attributes deprecated in Html5.
You must use css border properties to style them ^^

Answer 2

one works and the second is an error!

Answer 3

you always need a unit in css (exept 0 cause it's always 0) if you don't specify one, to (clever) browsers take it as pixel, but it's definetly invalid code

Answer 4

Thanks #kamil & #visph