[問題] w3school講的DTD是什麼意思?

[lucasbard (路卡思詩人)]

An HTML document is validated against a Document Type Definition (DTD).
Before an HTML file can be properly validated, a
 correct DTD must be added as the first line of the file.

這一段英文好玄  請知道的人幫我解釋一下好嗎?
謝謝


網頁來自:
http://www.w3schools.com/html/html_whyusehtml4.asp

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