Bert Bos,
style activity leadCopyright © 1994–2022 W3C® Privacy policy
The W3C Style page contains a <blink> element. <blink> is not an HTML element and yet the page is valid according to some HTML validators. What's the trick?
In fact, the page is syntactically valid, because the element is declared in the document. As long as elements are properly declared, the page is valid in the SGML/XML sense.
But the page is not semantically valid. It is an SGML document, but it is not an HTML document. There is no official standard that defines what <blink> means.
The W3C Style page uses a non-standard element as a joke. If you know the history of CSS you may remember that one of the reasons for CSS was precisely to avoid that every browser invented its own non-standard elements, such as <blink>.
Technically, adding non-standard elements is done by writing a “custom DTD” and including it in the first line of the document (the DOCTYPE line). A custom DTD can be made, for example, by copying the HTML4 DTD and modifying it.
Don't do this! Documents need to have a meaning as well as correct syntax. SGML and XML only define syntax. HTML and XHTML define meaning. If you add elements that aren't defined by a standard, only you yourself know what they mean. And in 20 or 50 years, even you may not know it anymore…
Of course, you can experiment, for example to work on future Web formats, but other than that you should not use proprietary elements.
Bert Bos,
style activity lead