I\'m hopeless at Javascript. This is what I have:
HTML5 does not support the tag, however a lot of browsers will still display the text "properly" but your code will not validate. If this isn't an issue for you, that may be an option.
CSS3 has the ability, supposedly, to have marquee text, however because anyone that knows how to do it believes it's a "bad idea" for CSS, there is very limited information that I have found online. Even the W3 documents do not go into enough detail for the hobbyist or self-teaching person to implement it.
PHP and Perl can duplicate the effect as well. The script needed for this would be insanely complicated and take up much more resources than any other options. There is also the possibility that the script would run too quickly on some browsers, causing the effect to be completely negated.
So back to JavaScript - Your code (OP) seems to be about the cleanest, simplest, most effective I've found. I will be trying this. For the seamless thing, I will be looking into a way to limit the white space between end and beginning, possibly with doing a while loop (or similar) and actually run two of the script, letting one rest while the other is processing.
There may also be a way with a single function change to eliminate the white space. I'm new to JS, so don't know off the top of my head. - I know this isn't a full-on answer, but sometimes ideas can cause results, if only for someone else.