The reason you lost those characters was, as Grishka hinted, that you included a "<" sign in the ABC source and it was interpreted as the start of a (nonexistent) HTML tag - you need to replace all occurrences of < with "&lt;". Also, if you use monospace to make the tune's metrical structure more readable as I do, you need to replace every space character with "&nbsp;" and enclose the tune in <tt> ... </tt> tags.