If you're reading about the <center> tag, you'll note the advice
The center element was deprecated in HTML 4.01.
The center element is not supported in XHTML 1.0 Strict DTD.
If you're not blocking popups from http://www.w3schools.com, you'll note a popup window advising you that
A deprecated element or attribute is one that has been outdated.
Deprecated elements may become obsolete in the future, but browsers should continue to support deprecated elements for backward compatibility.
In other words, you can keep using <center> ... </center> right now, but one day in the future, it may stop working. No guarantee when, either.
In HTML, an element is deprecated only when there is a replacement for it. In this case, this is a good replacement. The <center> ... </center> tags are known to display differently in Firefox and Internet Explorer.
For <center> ... </center>, you're advised
Use CSS styles to center text!
<span style="text-align:center;"> ... </span>to center text, for instance. You can use the
<span> ... </span>pair within a paragraph, or wherever else convenient. And if you want to center graphic objects, or multiple text objects, relative to each other, you can use nested tables.