Obsolete Tags in HTML 4.0
Obsolete tags have been removed from the HTML specification. While browsers may still support obsolete tags, there is no guarantee that this support will continue.
The three tags that become obsolete in HTML 4.0 are <XMP>, <PLAINTEXT>, and <LISTING>. In all cases, replace these tags with <PRE>.
NOTE: Despite the fact that the HTML 4.0 draft doesn't specifically mention frames as new, and despite the fact that you may have seen them in use for some time now, frames are new to the official HTML specification.
from Special Edition Using HTML 4: Appendix A
What's New in HTML 4.0
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