Starting game development in javascript. Where to draw the line between JS and Server Language
python would probably run on the server powering your web server.
If you want to write multiplayer client-server games then a lot of server code/logic would be in python.
If you want to write simpler singleplayer games then all your code is in the client (browser) and would be javascript. Python would just be there for serving your web game.
If you want to write desktop games you can do that in python using the pygame library.
So depends on what you want.

Multiplayer web game : client in JS, server in pythong
Singleplayer web game : client in JS, server only for serving web files
Desktop game : client in python.
Im fairly proficient in php and am also learning python. I have been wanting to create a basic game for some time now and would like to create it in python. But in order to have a fancy smooth interface I need to use javascript (i dont much care for the flash/silverlight route). So I decided to start looking up game development in JavaScript. And in doing so most of the examples I have seen are games nearly completely done in javascript. And many of these games are quite nice. 
But what im wondering, is if i combine python and javascript together for creating games; should I rely more heavily on javascript and let python do some of the background stuff? Or should I only use javascript for the basic UI elements, animation, flashy stuff and whatnot and keep the core game engine in python. Or some kind of mix between the two.
For php/python im mostly used to MVC style frameworks. So if i used JS only for the visuals would I just make heavy use of the in the view files as I would with a normal application? I know I can make a game in either language completely, but to use the best of both worlds im not quite sure where I should draw the line on letting python do work and letting JS do the work.
Also as a reference I would like to use some advanced features like canvas/svg and so on. Not really so much WebGL. And the game I have in mind to practice on is an isometric style RTS. The game will mostly be for learning. Im not really planning on releasing it.
Any input is appreciated.

Clarification:
the game will be web based. My web server is nginx. The games i would like to to would be multiplayer games where multiple people are playing online at the same time. Think world of warcraft. each server could hold up to N players that play against eachother. When that one is say 80% full a new one is spun up. Or something along those lines. I would like for the players to actually be able to interact with eachother live. Which is why in same ways i was also kind of looking into node.js but dont know as much about it yet. but seemed like it could fit.
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Update: Im also planning on using mongodb as the database, if that matters to anyones answer.