For a long time, the W3C has been encouraging everyone, developers and web browsers, to implement CSS to define HTML style, XHTML layouts.
This "center" element defines how certain content should be presented, while XHTML / HTML should define "document" - meaning, semantics - never "how to display content." This is the role of CSS.
This tag is supported for backward compatibility, but no one should use it in new web development, even knowing that CSS is easy, but sometimes centering some part of the layout requires advanced styling skills.
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