Warning: this post is old and might not reflect the current state of the art
<input autofocus="autofocus" /> will put the focus on the specified HTML element when the page loads
<a href="file.pdf" download="pdf-file">Download</a> will download the specified file, with the name provided.
<div hidden="hidden"></div> despite putting the presentation in the HTML, which is bad, this can sometimes turn out useful.
Operating systems configured to spell check everything a user types can sometimes get in the way - <input type="text" spellcheck="true|false"> can help.
<input list="mylist" name="mylist" >
<datalist id="mylist">
<option value="CSS">
<option value="HTML">
<option value="PHP">
<option value="Jquery">
<option value="Wordpress">
</datalist>