4.10 [CSS-Level1-2008] Appendix B, CSS1 grammar

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V0913:

The [CSS-Level1-2008] specification, Appendix B, states:

 nmchar           [-a-z0-9]|{latin1}|{escape}
 name             {nmchar}+
  
 "."{name}        {BEGIN(AFTER_IDENT); return CLASS;}

Quirks Mode (All Versions)

Class names that begin with an unescaped digit are allowed in all cases, even if it is a known dimension.

Note In [CSS-Level1-2008], a class name could start with a digit (for example, .55ft), unless it was a dimension (.55in). In [CSS-Level2-2009], such classes are parsed as unknown dimensions (to allow for future additions of new units). To make .55ft a valid class, [CSS-Level2-2009] requires the first digit to be escaped (for example, .\\35 5ft).

V0914:

IE7 Mode, IE8 Mode, and IE9 Mode (All Versions)

Class names that begin with an unescaped digit are not allowed in any case.