Executing "ng serve" or "npm start" command browser is not opening automatically

One developer of theangular-cli team said on Jun 17, 2016;

We don't have this functionality at the moment, no. But I'll leave the issue open as a 'nice to have'.

but

You could just change what npm start does in the package.json.

E.g. change

"start": "ng serve"

to

"start": "start http://localhost:4201 & ng serve -port 4201"

Per the documentation you can use a flag with ng serve:

--open (-o) Opens the url in default browser.

You may need update your version for this functionality, as the revision history of the wiki page suggests it's a recent addition.