With my solid foundation in web development, particularly HTML, CSS, and JavaScript, I am certain I can give your existing flight tracking HTML code the enhancements it needs while preserving its current functionalities. My nearly 2 years of experience enable me to work with meticulousness and creativity, qualities necessary to execute your desired improvements smoothly with minimal interference with existing features.
Speaking specifically on the first enhancement, adding a Head-Up Display (HUD), I've successfully integrated complex designs into websites before. Your specification for this feature is clear and feasible; including toggling on and off the display, incorporating various ladders (pitch, speed, altitude) that correspond to actual angles/movements and adding speed/altitude boxes. I'm confident that I can meet these specifications using the graphics provided by you.
Furthermore, for the second enhancement—creating a resizable time window for playback—I envision using three interactive sliders similar to those in video editing software. This involves adjusting start time, end time and current playback time. Although Cesium won't remain intact in this modification, my adeptness at assimilating new features while retaining the current ones will ensure a seamless transition. Trust my competence to update your HTML file with clean and maintainable code that will pass all necessary tests with flying colors. Let's get started!