Core APIs for common functions (intercept these network requests, inject these stylesheets, etc.) are being standardized so you don't have to reinvent the wheel

Core APIs for common functions (intercept these network requests, inject these stylesheets, etc.) are being standardized so you don't have to reinvent the wheel on every damn browser. Browsers can also freely implement additional, non-standard add-on APIs that make sense for their product, while the APIs that aren't distinguishing features can get out of the way and Just Work, everywhere. For example, Opera has a sidebar API, while Chrome does not. This is good, and allows browsers to compete on privacy, performance, security, support for web standards, ui/ux, etc.