The version history of the HTML language is listed below.
HTML 1 - 1991 (Basically Tim Berners-Lee's Original HTML)
HTML 2 - 1995
HTML 3 - 1997 (First Version Maintained by the W3C)
HTML 4 - 1998 (End of Browser Wars)
XHTML 1 - 2000
HTML 5 - 2014 (Current Version)
During the browser wars, the HTML langauge was expanding as the competing browsers, Netscape Navigator and Internet Explorer, invented new HTML features. By about the time of HTML 4, the Browser Wars were ending and the popular Web Browsers of the time started using (for the most part) the World Wide Web Consortium's HTML recommendations.
At about the same time, a new language named Cascading Style Sheets (CSS) was emerging. The goal was for CSS to provide stylistic control (colors, fonts, alignment, etc.) for Web pages, whereas HTML should only define content and structure as was its original purpose. That's a logical division of labor and, as you will see later in this course, brings added power and flexibility to Web developers.
More and more stylistic elements and attributes had been added to HTML as it had expanded prior to CSS. That is, HTML had gotten somewhat bloated and contained formatting features that should logically be part of CSS instead. When CSS became widely supported by browsers around 2000, the W3C needed to "purify" the HTML language and remove things that could be done using CSS. Toward that end, the W3C gradually began "slimming down" the HTML language. If it deals with color, font, size, width, alignment, or other purely stylistic things, it's probably been removed from HTML.
When W3C removes an element or attribute from HTML, it is called deprecated, meaning it's no longer part of the officially HTML language. However, Web browsers will still render deprecated elements/attributes years or even decades later. Since millions or billions of existing Web pages already use the deprecated elements/attributes, Web browsers can't simply quit supporting them or lots of Web pages would get messed up. So for the most part, current Web browsers still support all of the deprecated elements and attributes.
The first example of this lesson actually uses several deprecated attributes since the hr element's attributes are a great way to introduce the concept of HTML attributes. The Horizontal Rule is part of HTML 5, but ALL of its attributes have been deprecated. This screenshot from W3Schools hr reference page shows that. This is preserved as a screenshot because at some point W3Schools will probably delete all reference to these deprecated attributes.
It's important to understand this concept even if your objective is only to learn the most current HTML version. You will still run across deprecated HTML features in older Web pages. Some developers still use some deprecated HTML simply out of habit or convenience. The following centered stuff uses some deprecated attributes, but you will have to see the source code for explanation.
It's not "wrong" to use deprecated elements.
But most professional Web developers don't use them as a matter of good practice
and because CSS provides much better formatting potential anyway.
The last point in this example is the strange change to the name XHTML in 2000, which refers to eXtensible HTML. There is a technology named XML (eXtensible Markup Language) that was gaining in popularity around that time. Further discussion of XML is way beyond the scope of this course, but the comments in the source code explain some concepts related to XHTML of which you should be aware.