which most (all?) browsers would render as text separated by a line break
This isn’t strictly accurate – the paragraphs are separated by a margin or padding, which by common default happens to be approximately the height of a line of text. There is no actual line break (<br> element), just two paragraph elements, one after the other. The resulting HTML code could look like this:
<p>This is a paragraph of text. It is pretty boring</p><p>This is a second paragraph, similarly devoid of interest.</p>
Moreover, this spacing is entirely under the control of the page’s CSS, so you can’t always expect it to be present – some sites, for example, use book-style paragraps (no spacing, but indenting the first line).
I think it’s important to remember this – Markdown is just for creating HTML; how that HTML is presented can vary widely depending on the CSS applied to it.