Are you asking for this functionality in the editor or browser or something else? Editors do highlight text as you describe, at least in Emacs in markdown-mode

Are you asking for this functionality in the editor or browser or something else? Editors do highlight text as you describe, at least in Emacs in markdown-mode and org-mode. Still, my experiments with variable width fonts in these modes have never stuck. As soon as you need to align anything like a header rule, source code, or table, you need a fixed width font. Then it comes down to whether the mode supports customizing that particular face.