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Amateurs refer to XHTML elements are "tags" and software developers refer to them as "nodes". You're posting in the wrong forum as you're very likely asking how to get the textNode from the header.
Do not use innerHTML unless it's for backwards compatibility purposes. Learning how to properly work with "nodes" in JavaScript can be greatly rewarding in the long term if you come to realize what you can eventually do with it.
Granted innerHTML is faster however it's nearly impossible to work with (X)HTML loaded via innerHTML because it's not added to the DOM. Therefor you may script something and it may or may not appear inside the DOM. I'm just trying to make sure this is understood so lovelf does not end up wondering down the road why they can't script this or that.
I've had numerous difficulties accessing elements by their ID with innerHTML which is why I've mentioned it. It half-works and half doesn't. It really depends on your goal(s), approach, etc as to whether or not you encounter this error or not. There is always the possibility that innerHTML used to not work with the DOM as desired and now does with newer versions of given rendering engines. Still I highly recommend working with JavaScript nodes however when you need to do a simple write without any desire to read to a containing element I have nothing against using innerHTML.
Somewhere I've read that they've scrapped DOM level 3, haven't heard of DOM4, and that DOM5 is only what the browser vendors have implemented...as if more subjectivity were truly desired on the topic!