Xhtml functionality in Html file

Firefox and Internet Explorer appear to look at the incoming file tag to decide whether to treat the file contents as Html or Xhtml. Using Rails, I’d like to make templates that include Xhtml elements having MathML and SVG. Innocently, I thought I could get the browser to think Xhtml by finding the right DOCTYPE and META lines within a file having tag html.
I still think that must be right, but I haven’t figured out how to write the DOCTYPE and /or META lines.
All ideas will be welcome.

To be clear: A file opened by Firefox containing an <svg> element will render the circle on the browser page PROVIDED the file’s tag is Xhtml or Xml. Say, ‘circle demo.xhtml’ but if the file tag is changed ‘circle demo.html’ then Firefox will not render the circle.
Thanks.
Bob Barry