2.2 Undefined Tag and Attribute Names

An accepted networking principle is to be conservative in that which one produces, and liberal in that which one accepts. HTML user agents should be liberal except when verifying code. HTML generators should generate strictly conforming HTML.

The behavior of HTML user agents reading HTML documents and discovering tag or attribute names which they do not understand should be to behave as though, in the case of a tag, the whole tag had not been there but its content had, or in the case of an attribute, that the attribute had not been present.