21353 2013-03-20 17:34:28 +0000 While I can't find it on this page, presumably the valid characters in an attribute value does not include : So wherever the spec is for valid chars in attribute value, that's the answer for this one. 2013-04-14 06:51:34 +0000 1 1 1 Unclassified WHATWG HTML unspecified Other other RESOLVED NEEDSINFO http://www.whatwg.org/specs/web-apps/current-work/#syntax-attribute-value P3 normal Unsorted 1 contributor ian ian mike contributor oldest_to_newest 84746 0 contributor 2013-03-20 17:34:28 +0000 Specification: http://www.whatwg.org/specs/web-apps/current-work/multipage/syntax.html Multipage: http://www.whatwg.org/C#syntax-attribute-value Complete: http://www.whatwg.org/c#syntax-attribute-value Comment: While I can't find it on this page, presumably the valid characters in an attribute value does not include : So wherever the spec is for valid chars in attribute value, that's the answer for this one. Posted from: 67.162.103.133 User agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.6; rv:19.0) Gecko/20100101 Firefox/19.0 85290 1 ian 2013-04-01 18:27:49 +0000 Nope, colons are fine in general. What exactly is allowed in attribute values depends on the attribute and on the syntax used for the attribute. For example, this is invalid: title=can't do this ...because apostrophes and spaces aren't valid in unquoted attribute values. On the other hand, this is fine: title="y'all CAN do this" ...because those characters are fine when quoted. But it depends on the quotes; this isn't valid, for example: title='can't do this' But some attributes are even more constrained, e.g.: dir="can't do this" ...is invalid because dir="" only takes a few values like "ltr" and "rtl". The text at the place in the spec where you were gives the broad requirements: http://www.whatwg.org/C#syntax-attribute-value Is that insufficiently clear?