div.classname in css file

Question

I have seen some developers write

HTML:

 some content 

CSS:

Answer 1

Does this mean this style will be applied only when the class is applied to a div ?
Yes

The previous rule will not in deed have any effect on span

The reason for such choice depends on the context. In general, any css style is to focus on a particular element on a given page otherwise you will have

div { .... }