IE6 is below 5% market share. But if you absolutely have to support IE6, then yeah, use JQuery.

There are still some differences in modern browsers but those can mostly be repaired with polyfills.

Oh, and while I would not recommend doing your chaining operation (chaining just makes it harder to debug), I can write the same thing in one line of JavaScript:

      Array.prototype.slice.call(document.querySelectorAll('.something > *')).forEach(function(el) { el.className += ' bold-font'; });