<!DOCTYPE>標籤必須放在整個html文檔的第一行，之後一行就是從<html>標籤開始，所有瀏覽器都支持<!DOCTYPE>標籤。
<!DOCTYPE>標籤是用來告訴瀏覽器用哪個html版本解析當前的html頁面。
其中很多DOCTYPE中都帶有DTD聲明，DTD就是html文檔的規範說明，主要有：strict（嚴格）、transitional（過度）、frameset（框架），嚴格類型的DTD包含了大部分的html元素和屬性，但一些過時的元素已經不再支持（font\u\s\iframe等）；過度的包含了所有的html元素和屬性，過時的一些元素也支持；框架的DTD是爲需要使用框架時所用到的DTD，如無框架，不會用到。
常用的html版本對應的DOCTYPE聲明：
| html版本 | DOCTYPE聲明 |
| HTML 5 | <!DOCTYPE html> |
| HTML 4.01 Strict | <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd"> |
| HTML 4.01 Transitional |
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" |
| HTML 4.01 Frameset |
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Frameset//EN" |
| XHTML 1.0 Strict |
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" |
| XHTML 1.0 Transitional |
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" " |
| XHTML 1.0 Frameset |
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Frameset//EN" |
| XHTML 1.1 |
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd"> |
對應與不同html版本的DOCTYPE所支持的元素是不同的，基本的支持情況如下列出。
| 標籤 | HTML5 | HTML 4.01 / XHTML 1.0 | XHTML 1.1 | ||
|---|---|---|---|---|---|
| Transitional | Strict | Frameset | |||
| <a> | Yes | Yes | Yes | Yes | Yes |
| <abbr> | Yes | Yes | Yes | Yes | Yes |
| <acronym> | No | Yes | Yes | Yes | Yes |
| <address> | Yes | Yes | Yes | Yes | Yes |
| <applet> | No | Yes | No | Yes | No |
| <area> | Yes | Yes | Yes | Yes | No |
| <article> | Yes | No | No | No | No |
| <aside> | Yes | No | No | No | No |
| <audio> | Yes | No | No | No | No |
| <b> | Yes | Yes | Yes | Yes | Yes |
| <base> | Yes | Yes | Yes | Yes | Yes |
| <basefont> | No | Yes | No | Yes | No |
| <bdi> | Yes | No | No | No | No |
| <bdo> | Yes | Yes | Yes | Yes | No |
| <big> | No | Yes | Yes | Yes | Yes |
| <blockquote> | Yes | Yes | Yes | Yes | Yes |
| <body> | Yes | Yes | Yes | Yes | Yes |
| <br> | Yes | Yes | Yes | Yes | Yes |
| <button> | Yes | Yes | Yes | Yes | Yes |
| <canvas> | Yes | No | No | No | No |
| <caption> | Yes | Yes | Yes | Yes | Yes |
| <center> | No | Yes | No | Yes | No |
| <cite> | Yes | Yes | Yes | Yes | Yes |
| <code> | Yes | Yes | Yes | Yes | Yes |
| <col> | Yes | Yes | Yes | Yes | No |
| <colgroup> | Yes | Yes | Yes | Yes | No |
| <command> | Yes | No | No | No | No |
| <datalist> | Yes | No | No | No | No |
| <dd> | Yes | Yes | Yes | Yes | Yes |
| <del> | Yes | Yes | Yes | Yes | No |
| <details> | Yes | No | No | No | No |
| <dfn> | Yes | Yes | Yes | Yes | Yes |
| <dir> | No | Yes | No | Yes | No |
| <div> | Yes | Yes | Yes | Yes | Yes |
| <dl> | Yes | Yes | Yes | Yes | Yes |
| <dt> | Yes | Yes | Yes | Yes | Yes |
| <em> | Yes | Yes | Yes | Yes | Yes |
| <embed> | Yes | No | No | No | No |
| <fieldset> | Yes | Yes | Yes | Yes | Yes |
| <figcaption> | Yes | No | No | No | No |
| <figure> | Yes | No | No | No | No |
| <font> | No | Yes | No | Yes | No |
| <footer> | Yes | No | No | No | No |
| <form> | Yes | Yes | Yes | Yes | Yes |
| <frame> | No | No | No | Yes | No |
| <frameset> | No | No | No | Yes | No |
| <h1> to <h6> | Yes | Yes | Yes | Yes | Yes |
| <head> | Yes | Yes | Yes | Yes | Yes |
| <header> | Yes | No | No | No | No |
| <hgroup> | Yes | No | No | No | No |
| <hr> | Yes | Yes | Yes | Yes | Yes |
| <html> | Yes | Yes | Yes | Yes | Yes |
| <i> | Yes | Yes | Yes | Yes | Yes |
| <iframe> | Yes | Yes | No | Yes | No |
| <img> | Yes | Yes | Yes | Yes | Yes |
| <input> | Yes | Yes | Yes | Yes | Yes |
| <ins> | Yes | Yes | Yes | Yes | No |
| <keygen> | Yes | No | No | No | No |
| <kbd> | Yes | Yes | Yes | Yes | Yes |
| <label> | Yes | Yes | Yes | Yes | Yes |
| <legend> | Yes | Yes | Yes | Yes | Yes |
| <li> | Yes | Yes | Yes | Yes | Yes |
| <link> | Yes | Yes | Yes | Yes | Yes |
| <map> | Yes | Yes | Yes | Yes | No |
| <mark> | Yes | No | No | No | No |
| <menu> | Yes | Yes | No | Yes | No |
| <meta> | Yes | Yes | Yes | Yes | Yes |
| <meter> | Yes | No | No | No | No |
| <nav> | Yes | No | No | No | No |
| <noframes> | No | Yes | No | Yes | No |
| <noscript> | Yes | Yes | Yes | Yes | Yes |
| <object> | Yes | Yes | Yes | Yes | Yes |
| <ol> | Yes | Yes | Yes | Yes | Yes |
| <optgroup> | Yes | Yes | Yes | Yes | Yes |
| <option> | Yes | Yes | Yes | Yes | Yes |
| <output> | Yes | No | No | No | No |
| <p> | Yes | Yes | Yes | Yes | Yes |
| <param> | Yes | Yes | Yes | Yes | Yes |
| <pre> | Yes | Yes | Yes | Yes | Yes |
| <progress> | Yes | No | No | No | No |
| <q> | Yes | Yes | Yes | Yes | Yes |
| <rp> | Yes | No | No | No | No |
| <rt> | Yes | No | No | No | No |
| <ruby> | Yes | No | No | No | No |
| <s> | Yes | Yes | No | Yes | No |
| <samp> | Yes | Yes | Yes | Yes | Yes |
| <script> | Yes | Yes | Yes | Yes | Yes |
| <section> | Yes | No | No | No | No |
| <select> | Yes | Yes | Yes | Yes | Yes |
| <small> | Yes | Yes | Yes | Yes | Yes |
| <source> | Yes | No | No | No | No |
| <span> | Yes | Yes | Yes | Yes | Yes |
| <strike> | No | Yes | No | Yes | No |
| <strong> | Yes | Yes | Yes | Yes | Yes |
| <style> | Yes | Yes | Yes | Yes | Yes |
| <sub> | Yes | Yes | Yes | Yes | Yes |
| <summary> | Yes | No | No | No | No |
| <sup> | Yes | Yes | Yes | Yes | Yes |
| <table> | Yes | Yes | Yes | Yes | Yes |
| <tbody> | Yes | Yes | Yes | Yes | No |
| <td> | Yes | Yes | Yes | Yes | Yes |
| <textarea> | Yes | Yes | Yes | Yes | Yes |
| <tfoot> | Yes | Yes | Yes | Yes | No |
| <th> | Yes | Yes | Yes | Yes | Yes |
| <thead> | Yes | Yes | Yes | Yes | No |
| <time> | Yes | No | No | No | No |
| <title> | Yes | Yes | Yes | Yes | Yes |
| <tr> | Yes | Yes | Yes | Yes | Yes |
| <track> | Yes | No | No | No | No |
| <tt> | No | Yes | Yes | Yes | Yes |
| <u> | No | Yes | No | Yes | No |
| <ul> | Yes | Yes | Yes | Yes | Yes |
| <var> | Yes | Yes | Yes | Yes | Yes |
| <video> | Yes | No | No | No | No |
| <wbr> | Yes | No | No | No |
No |