X509 Cert exploit CA check
When you connect to a secure site the certificate validation happens. Certificate validation failure can happen when:


Fields CN or SAN dont't match the site's domains. That means that the certificate was issued for a different domain.
Invalid digital signature. Certificate has been altered.
Unknown certificate issuer, thus unknown intermediate CA or root CA.
Other common errors such expiration, revocation etc


Notice that an error here will warn the end-user via the browser. So the end-user has the power to trust a forged certificate if possible.

Now, in the case that you suggest, if the browser skips the root CA validation, it will only validate the digital signature in the certificate chain validation process, so it stops when reaching the CA Root. This means that the end-user will only validate digital signatures according to the certificate's Issuer public key until you hit the root CA, then process stops here without checking the key ring/trusted anchors.

This has happened in some crypto libraries among non-browser applications allowing an attacker to MITM a TLS session with forged certificates. Take a look at https://www.cs.utexas.edu/~shmat/shmat_ccs12.pdf