These two criteria are important, because they help screen readers to read out the language correctly. If your page is set to English, but some of your content is not English, then the screen reader is just going to attempt to pronounce it in English anyway, which can lead to some pretty weird results.
As designers, we always like to put our stamp on things. We like to make things fancy and show off our full range of talents. Then when it comes to coding them up, we abuse our design!
We float things right. We use absolute positioning. We style links to look like buttons. We use fancy hover states and chuck in break tags in to create whitespace.
Then, we marvel at how pretty our designs look. After all, as long as it looks good, that's all that matters. Right?
I've recently been ranting a lot about primary buttons. But people often don't understand what they are.
In my early days as a designer, I thought it just meant adding class="btn-primary" to my markup to make it pretty and blue, and this is a common mistake.
A lot of people make this mistake and litter the Internet with bad design which is confusing for the people that have to use it. Some of my early contributions are still out there causing a nuisance. Sorry.