Just to expand on what Silent said...
C++ is typically compiled to machine code and therefore generally has no run-time dependencies except for any dynamically linked libraries. Various Python implementations may or may not have more dependencies (e.g. an interpreter and/or run-time support components).
Web browsers natively support HTML and most also natively support Javascript and CSS. I do not know of any that natively support Python, although you can certainly use Python on a server to generate browser-supported content. In theory, you could also add native Python support to browsers supporting plugins, but I suspect you would have significant adoption issues unless you're dealing with a closed, controlled user population (perhaps a corporate intranet).
I believe Java is the most widely supported general programming language with browser plugin support, however a number of sources (including the US government) are currently recommending disabling browser-side Java support whereever possible due to security concerns.
A common situation for many web application developers is to generate HTML, CSS, and Javascript for the client-side (browser) code in conjunction with some other language (ASP, C#, Java, Perl, PHP, Python, Ruby, etc.) on the server.
I hope that helps.