Assertion:
Nested MARQUEE elements do not behave well in Internet Explorer. Don't try it.

Basic nested MARQUEE elements, no widths on each. Green background on outer, yellow on inner:
filler text outerMarquee innerMarquee outerMarquee filler text

Basic nested MARQUEE elements, width=500 on each. Green background on outer, yellow on inner:
filler text outerMarquee innerMarquee outerMarquee filler text