Responsive web development has become common place and that's a good thing, developers are taking notice of not just device size but feature support as well. However, I still find myself fixing the same bugs again and again because people haven't started building responsively for content.
You must only have one <H1> on a page! The words of "SEO experts" everywhere and the starting point of many tedious discussions about best practise. I often get warned about the penalties of multiple H1 tags and decided it was time I put together some numbers.