No margin-bottom on bootstrap button?

Just add the spacing class. m for margin and p for padding.

 <button class="btn btn-success mr-2" type="submit">Add</button>
 <button class="btn btn-danger mr-2" type="button">Delete</button>

so, here mr is margin-right. similarly, you can add do for other cases.

I think that it's the normal behavior ( not 100% sure ) because it's "cleaner" to add margin ourselves when we want it that having to remove the default margin when we don't want it, from my opinion...

http://getbootstrap.com/components/#btn-groups

There's no margin on bootstrap's buttons examples, only on the parent div class btn-group but it's coming from a different stylesheet ( docs.min.css ). So I think it's that way for devs to add their own custom margins.

See here:

In Bootstrap's buttons.less and button-groups.less there's nothing about margin-top or margin-bottom. Having a margin by default would likely conflict when combining it with other elements (e.g. a form)

I think the best solution might be adding all buttons inside a btn-toolbar and to style that combination:

.btn-toolbar .btn {
    margin-bottom: 5px; 
}

mb-5 = margin bottom ..just add that :) sorted :)