data

Data URLs let small document data be included in the URL itself. This is useful for very small HTML test cases, or other occasions that don't justify a document of their own.

data:,foobar (note the comma after the colon) delivers a text document that contains nothing but foobar.

That example delivered a text document. For HTML documents, you have to specify the MIME type text/html: data:text/html,<title>Testcase</title><p>This is a testcase</p>. This produces exactly the same output as if the content had been loaded from a document of its own.

You can also specify other character sets. Note that 8-bit characters have to be escaped by a percent sign and their two-digit hexadecimal codes: data:;charset=iso-8859-1,Gr%FC%DFe aus Schl%E4gl results in Grüße aus Schlägl, whereas omitting the charset attribute might lead to something like Gr??e aus Schl?gl.

IETF RFC 2397 has more information.