No and yes. Current browser releases do not speak XHTML, they speak Tag Soup masquerading as HTML (and HTML too, but that's by accident). However, it is possible to write XHTML in a way that is compatible with HTML, and will not break on existing HTML-based browsers. These are described in Appendix C of the XHTML specification.
As an example, <HR> is an empty element, and would
normally be written in XHTML as either <hr/> or
<hr></hr>.
Either will confuse many existing browsers; instead, you can write
the tag as <hr /> - note the space. This is
legitimate XHTML and is compatible with HTML browsers.