Solution to using margin:0 auto in html and the entire page is not centered_HTML/Xhtml_Webpage Production

Today I am writing a jsp page, and I have been adjusting it repeatedly:

123xxx

No matter how I adjust this property, I can't get the page to be displayed in the center, and other styles also have inexplicable problems

Later I found a solution to this problem:

It turns out that L-Blog does not add DTD before HTML by default, so IE interprets the document as HTML instead of XHTML.
Problem It is not in the CSS but in the XHTML web page itself.
You need to add this code to make the above settings effective:

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The code is as follows:

If you want more strict XHTML 1.0 Strict or XHTML 1.1, please consult the relevant documentation.
The above tests are based on Windows XP SP2 version IE6 and FireFox 1.0 final version.

It is recommended not to delete this line if you need to use css styles in the page:

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The code is as follows: