Limitations of Selenium Web-driver. ( Browser's Native Support)

Selenium web-drivers operates at Operating System Level.
Now, each browser communicates with its Operating System in its own way.
The way by which browsers communicates with the Operating System is called Browser's Native support or its is browser's built in support for automation.
When any new browser comes into market then we will have to give some time to selenium web-driver to understand the browser's native support.
So web-driver cannot readily support new browser.
HTMLUNIT and Firefox are the two browsers that web-driver can directly automate.
For other browsers it require Driver server.