I want to create an animated HTML \"marquee\" that scrolls back and forth on a website:
This is a marquee! >
This is a marquee!
If I understand you question correctly, you could create a wrapper around your marquee and then assign a width (or max-width) to the wrapping element. For example:
width
max-width
And then #marquee-wrapper { width: x }.
#marquee-wrapper { width: x }