Porcelain mugs are better than ceramic mugs for coffee as porcelain coffee mugs hold heat in better than ceramic mugs so your coffee will stay hotter longer.
Ceramic travel mugs are safe as long as they are cared for properly and you avoid dropping them.
Ceramic travel mugs do break easily if you're not careful with them although ceramic travel mugs are also very durable and are generally safe when handled properly.
Stainless steel travel mugs are much safer and are more durable and don't break like ceramic mugs can.
Also the stainless steel travel mugs hold in the heat better so your coffee or other hot drink would stay hotter longer than it would in a ceramic travel mug.
The thing at the bottom of Ikea coffee mugs is a drain called drainage gates.
The tiny hole at the bottom of the Ikea coffee mugs is to drain any excess water after the Ikea coffee mugs are washed.
These drainage holes or drainage gates are meant to transport any water away from the coffee mugs which would otherwise accumulate when the coffee mug is upside down in the dishwasher.
A small chip at the bottom of most coffee mugs is called a "kiln mark" or "firing pin mark."
The Kiln mark on the coffee mug is a result of the manufacturing process, where the mug is placed in the kiln and supported by small prongs to prevent the glaze from sticking to the kiln shelf.
A coffee mugs bottom is often not flat, but either concave or has an extra rim, to reduce the thermal contact with the surface on which a mug is placed.
These features often leave a characteristic circular stain on the surface.
And the handle of a mug keeps the hand away from the hot sides of a mug.