You could try ionic serve --browser="chrome" or another browser as target (safari, firefox).
If your browser does not open, try to open http://localhost:8100/manually in your browser.
For the warning with PostCss you find the solution here: Pls help me sir
If Nexi’s suggestions don’t help, try running npm run ionic:serve and see if this works. If it does, please report an issue for ionic-cli on Github so this can get fixed. Thank you.