That's only partly correct.
To work in all browsers, the border shorthand property requires 3 attributes:
- width (medium, thin, thick, or a number value in px or em),
- style (dashed, dotted, double, solid, inset, outset, groove, ridge or none)
- color (either #hex or rgb).
This is valid code:
border: 2px solid #000;
And this is valid:
border: thin dotted #000;
For more on CSS Borders:
Alt-Web Design & Publishing
Web | Graphics | Print | Media Specialists
I think only the border-style property is required, the others will fall back to default values.
From the page you linked to:
"It does not matter if one of the values above are missing (although, border-style is required), as long as the rest are in the specified order."
Also see number 3 here.
But I agree, it's a good idea to get in the habit of specifying all three.