How to include a html page into another html page?

Hi all,
I am a newbie to HTML includes.

I want to include a header.html page into my test.html, test1.html.

When i change header.html, it will immediately reflect the changes in the test.html, test1.html.

How can i include like this functionality?

Give me your suggestions…

Thanking you…

using a server side language, for example PHP

not by using php…

When i include by using this code in a test.html;

<!–#include file=“header.html” –>

After i see the source of html in the browser, it is showing two <html>, two <head> like that…

How can i prevent this…
Give me the pros and cons if i use like this…

Thank you…

Get rid of the <html> and <head> in one of the files (the main html or the included html). The end result must be a complete, valid html page, so you’ll have to put each piece in the right file.