The problem is that every browser renders fonts just a little differently, with small differences in character width, spacing, and line wrapping. The RP9 editor renders fonts differently than most browsers… So, the longer your text string, the more these differences are evident, especially at newline breaks. This issue varies with each proportional font–some better, some worse. It shouldn’t be an issue with a fixed-width font, but most people don’t like reading those fonts.
This recent thread might give you some workarounds: