To my knowledge Lightpanda is a headless browser used for automation. While Zero Browser is just a UI for end user. Zero might be able to use Lightpanda as a optional plugin.
In simple terms it's built as a experimental implementation for a web ecosystem where websites hosts a standardized API endpoints and users are responsible for displaying it's contents. What AI almost does now anyway.
Before that happens it continues to support browsing static html pages and API's without using JS/CSS.