The hidden attribute, as you can easily guess, hides the element that it is set on. It acts almost identical to style
See the Pen Not so popular HTML5 attributes : hidden attribute by John Tsevdos (@tsevdos) on CodePen
Do you have downloadable content in your website (like PDF files etc)? If yes then with a little HTML5 attribute you can provide better semantics to the link element, give a better description/title to your file and enforce the browser to download it! Just use the download attribute followed by a string that represents the description/title of your download. Even if your filename is a timestamp or a temp name, the user will download the file, having as its name the title you used.
See the Pen Not so popular HTML5 attributes : download attribute by John Tsevdos (@tsevdos) on CodePen
Finally with the autofocus attribute you can autofocus any
button you want. Simple as that.
See the Pen Not so popular HTML5 attributes : autofocus attribute by John Tsevdos (@tsevdos) on CodePen
To conclude, whenever possible, try to use the new HTML5 attributes. They will always be more semantically correct and it’s super easy to implement them. Also, most of them have very simple fallbacks but even if you don’t provide any, most of the time you won’t break something, so you don’t have any excuse.