What does a colon mean within an HTML id attribute?

I have seen an HTML code like this for the ID attribute:

id="t1:c3"

Can someone explain this to me? What is the purpose of the colon (:)?

Thank you.

Colons are allowed inside ID attributes, but hold no special significance. It's not really advisable to use them because they can sometimes cause problems, such as when used with jQuery or CSS, where the colon has special meaning as a pseudo-selector.

If the HTML is generated with Java ServerFaces this usually produces such IDs, i.e. if the form has the ID form1 and the element has the ID firstName this results in an ID form1:firstName if the element is on top level, if there are several ones created in a loop they look like form1:1:firstName, form1:2:firstName, …

This framework behaviour cannot be changed.

In addition to the above I have discovered that Omniture uses colons to help define classes related to the layout.