note that only returns inputs, so if you have any textarea or select, you have to do multiple passes. This is why they recently gave us querySelectorAll.
container.querySelectorAll("input, textarea, select")
Will give you an iterable of all three tags.
getElementById, getElementsByClassName, and getElementsByTagName are fastest when you're looking for single matches. querySelector and querySelectorAll are "better" when you want to grab bunch of complex lookups possible results.
You might not even need the container. Let's say your container was <fieldset id="about">
aboutElements = document.querySelectorAll("#about input, #about textarea, #about select");