Main Thread

The main thread is the primary "workspace" within a web browser where it performs most of the work required to display a webpage. This includes parsing HTML and CSS, executing JavaScript, and responding to user interactions. If the main thread is busy for too long with a heavy script, it becomes "blocked" and cannot respond to user clicks or taps, leading to a laggy, unresponsive experience and a poor Interaction to Next Paint (INP) score.