The RandomSource.getRandomValues() method lets you get cryptographically random values. The array given as the parameter is filled with random numbers (random in its cryptographic meaning).
getDefaultComputedStyle() gives the default computed values of all the CSS properties of an element, ignoring author styling. That is, only user-agent and user styles are taken into account.
The window.postMessage method safely enables cross-origin communication. Normally, scripts on different pages are allowed to access each other if and only if the pages that executed them are at locations with the same protocol (usually both https), port number (443 being the default for https), and host (modulo document.domain being set by both pages to the same value). window.postMessage provides a controlled mechanism to circumvent this restriction in a way which is secure when properly used.
This method is used to break up long running operations and run a callback function immediately after the browser has completed other operations such as events and display updates.