Deprecated tags and attributes are those that have been replaced by other, newer, HTML constructs. Deprecated tags are still included in the HTML draft or recommendation but are clearly marked as deprecated. Once deprecated, tags may well become obsolete. The draft “strongly urges” the nonuse of deprecated tags.
<ISINDEX> allowed a form to contain a simple string search. This action should be replaced by an <INPUT> form element.
The <APPLET>...</APPLET> tag enabled the running of a Java applet. This tag has been replaced by the more encompassing <OBJECT>...</OBJECT> tag.
The <CENTER>...</CENTER> tag, oddly enough, centered text or graphics. <CENTER> is deprecated in favor of <DIV> tag with the align attribute set to “center.”
<FONT>...</FONT>allowed the specification of font sizes, colors, and faces. Style sheets, rather than HTML code, have taken over character formatting duties.