Proprietary Element

The <blink> element was introduced by Microsoft as a proprietary feature of Internet Explorer. It was never a part of the HTML specification. Today, it is not supported at all, even by IE.

With CSS: text-decoration: blink

An alternative way to accomplish this (ill-advised) effect was to use set text-decoration: blink; in CSS. Fortunately, this is not part of the CSS specification either, and does not work on most browsers.

If you really, really have to

If you need to cause text to blink in and out, you are probably doing something awful. It is an extremely annoying effect. That being said, you can accomplish it with JavaScript.

<span id="blinker">Blink! Blink! Blink!</span>
var blink_speed = 500;
var t = setInterval(function () {
    var ele = document.getElementById('blinker'); = ( == 'hidden' ? '' : 'hidden');
}, blink_speed);
Blink! Blink! Blink!

JS Code by Chris Baker, via Stack Overflow. Licensed in the Public Domain.