The zero width space is a useful Unicode character. It's white space but renders with zero width. Useful for hinting where a line break could go if a browser needs to wrap a long line. It's also good for faking out Twitter's annoying URL rewriter; if you stick a ZWS in the middle of a domain name then Twitter won't rewrite your text with a t.co redirect.
The zero width space is Unicode character U+200B. (HTML ​). It's remarkably hard to type. On Windows you can type Alt-8203. On Linux you apparently can type Ctrl-Shift-U 8203. On a Mac you need Character Viewer; search for "zero" and double click the invisible character on row 4, column 1 to insert a ZWS.
ZWS >< ZWSOr you can just cut and paste it. I put one up there for you, between the left and right angle brackets. Of course being zero width you can't easily select it; best bet is to copy the angle brackets too, all 3 characters. Then paste and delete the brackets. You can verify the ZWS is still there by using the arrow keys to move the cursor; it should get "stuck" on the ZWS and require two movements to pass.