photo via flickr (credit)
BR generates a line-break and it is only a line-break. As this element has no content, there are only few styles that make sense to apply on it, like
position. You can set
BR‘s border but you won’t see it as it has no visual dimension.
If you like to visually separate two sentences, then you probably want to use the horizontal ruler which is intended for this goal. Since you cannot change the markup, I’m afraid using only CSS you cannot achieve this.
It seems, it has been already discussed on other forums. Extract from Re: Setting the height of a BR element using CSS:
[T]his leads to a somewhat odd status for BR in that on the one hand it is not being treated as a normal element, but instead as an instance of A in generated content, but on the other hand it is being treated as a normal element in that (a limited subset of) CSS properties are being allowed on it.
I also found a clarification in the CSS 1 specification (no higher level spec mentions it):
The current CSS1 properties and values cannot describe the behavior of the ‘BR’ element. In HTML, the ‘BR’ element specifies a line break between words. In effect, the element is replaced by a line break. Future versions of CSS may handle added and replaced content, but CSS1-based formatters must treat ‘BR’ specially.