Here's my take on working with classes:
*Note that I'm not positive about any of this, which is why I would like Webflow to publish an official document on the topic.
Webflow works by using classes and combo classes, which are, essentially, parent and child classes, because the second (or third of fourth) class that you apply to an element only exists in a dependent relationship with the first (parent) class.
This can be confusing, because CSS doesn't work this way – with CSS, you can create many classes independent of one another, and apply them to a single element without the parent-child dependency. So, with CSS, you can create a box shadow and apply it to a div or an image independent of other attributes, such as padding and position.
Webflow's method can also cause a bit of extra work, because, if you want to create a consistent look (for example, use the same drop shadow on several different block elements), then you have to recreate the style for each different type of element. There is no way to save a shadow, so you have to remember each of the shadow values and enter them repeatedly.
A work-around in Webflow is to create a page just for creating "parent" classes. So, if you want to create a shadow that can be used on any block element, then create a miscellaneous page, create an element, give it a class name, and then configure the style that you want. Then you can apply this style as if it were a "parent," or independent, class.