I searched on Google but couldn't find an answer. I'd like to know what are the standard tab sizes for HTML and CSS. Is it 2 or 4?
Tabs have no innate size, their interpretation is up to the observer. If you're the observer, you get to decide how you want to view your tabs, or if you're using other kinds of whitespace, however many you want to have.
The syntax in the CSS specification uses
[ \t\r\n\f]+ (one or more of the bracketed things) for whitespace, implying that the kind of whitespace is irrelevant.
Similarly, HTML5 defines
space characters as:
Any of the following: U+0020 SPACE U+0009 CHARACTER TABULATION (tab) U+000A LINE FEED (LF) U+000C FORM FEED (FF) U+000D CARRIAGE RETURN (CR)