[user111233]

For developers it always seems like a good idea to use the system defaults but then you push it out to users and find out they all have insane defaults that work like crap.

I had someone complaining to me that my mailto: link doesn't work but actually its just they had the wrong mail client set as default on their system.

[qwerty456127]

Not using custom web fonts that have to be loaded from external resources doesn't imply using pure defaults of the client's system. You still can specify font size and style with pure CSS.