I found it fairly challenging to find a solution for this problem about margin space after a BR tag, so I thought I'd share what worked perfectly for me, with an addon / extension like Stylish for Chrome / Firefox, for example, and I'm guessing that it could be useful for CSS programmers / scripters too.
Of course, feel free to enhance or tweak if you're an expert!
br
{ content: "A" !important;
display: block !important;
margin-bottom: 1.5em !important;
}