Creating a "full fleshed out" 2D platformer that transitions to 3D after 10 levels, without any external dependencies and running purely in a browser environment, is an extremely complex task. Implementing physics, collision detection, level management, and especially a 3D rendering engine from scratch using only browser APIs (like Canvas for 2D and WebGL for 3D) requires a significant amount of code and effort, far beyond what can be provided in a single response.

However, I can provide a basic framework for a simple 2D platformer using HTML Canvas and pure JavaScript. This example will demonstrate fundamental concepts like player movement, gravity, jumping, and basic platform collision. The transition to 3D and the complexity of multiple levels and advanced game features are outside the scope of what's feasible to build from scratch in this format.

Here is a basic 2D platformer example:

This provides a very basic foundation. To make it a "full fleshed out" game with 10 levels and a 3D transition would require:

1. Advanced Physics: More realistic movement, friction, different jump heights, etc.
2. Level Design: Data structures to define multiple levels, loading/unloading levels, level progression logic.
3. Enemies/Obstacles: AI, collision with enemies, player health, death/respawn.
4. Collectibles: Items the player can pick up.
5. Camera: Scrolling the view as the player moves.
6. Sound/Music: Adding audio effects and background music.
7. User Interface: Score display, level number, pause menu, start screen, end screen.
8. 3D Engine: Implementing a 3D renderer using WebGL, including:
   • Loading 3D models or defining 3D geometry.
   • Setting up a 3D camera and projection.
   • Handling 3D transformations (translation, rotation, scaling).
   • Implementing 3D physics and collision detection.
   • Managing the transition logic from 2D to 3D.

Each of these points is a significant project in itself. Building a 3D engine from scratch without libraries is particularly challenging. This basic example focuses only on the core 2D movement and platform collision.*

Okay, let's modify the platformer to have procedurally generated platforms that appear above the player as they ascend, and ensure they are placed at jumpable heights. We'll implement a simple vertical scrolling mechanism where the camera follows the player upwards.

Now, the platforms will be generated above the current highest platform as you move upwards. The camera will follow you, giving the illusion of ascending. The vertical gap between platforms is controlled by minVerticalGap and maxVerticalGap, set to values that should be reachable with the current jumpPower.

Keep in mind this is still a basic implementation. A full game would require more sophisticated platform generation (e.g., ensuring horizontal reachability, varying platform types), level goals, enemies, etc. The 3D transition is a separate, much larger task involving WebGL and 3D math.