When geographically placing an HTML element on top of the Cesium canvas using cartesianToCanvasCoordinates to map a world position to canvas x and y values it has unexpected behavior when using Cesium World Terrain.
This is very noticeable when zoomed into a map really close (<200 m) and you tilt the camera the HTML element will move down the canvas out of view. Unlike when using WGS84 Ellipsoid it will stay in-place like it should.
Here’s when tilting the camera, notice the ceisum logo moves to the bottom.
The coordinates are also not updated correctly if you pan the map around.
Sandcastle link: HTML Element on Canvas with terrain example