Which of the two methods conforms to W3C standards? Do they both behave as expected across browsers?
Either is valid. Your choice but I would favour
border:0 as it's shorter. If you have a lot of traffic, you'll notice the difference!
You seem to be worried about the specs. Well here they are.
'border' Value: [ <border-width> || <border-style> || <'border-top-color'> ] | inherit Initial: see individual properties Applies to: all elements Inherited: no Percentages: N/A Media: visual Computed value: see individual properties
The value clearly states that you can use any combination of width/style/colour. In this case you only need to set one.
0 sets the width,
none the style.