In my experience its best to always include a closing tag. This is good for a few reasons:
If you are including your code into third party systems there is no telling what they are going to do to your code to make it valid.
E.G. in gmail you may find the second P tag to be completely removed whereas Outlook may close the first before the second P tag starts. Its best to keep things consistent.
Another example would be where Scrapers are generally going to need the HTML on the page to be as close to valid XML as possible in order to get the data they need.
It really depends on how you are using the code. My suggestion is to always close the tag but if for some weird reason you need to do it like that above just make sure to test test test... and test some more.
as a return line, alongside with
– Lord Rixuel Sep 27 '15 at 00:38. As for CSS, I prefer using id, class...