Examples: I know (about) this difficulty/problem. I know (about) Engineering. In the first sentence, it seems to me that "to know" expresses that the speaker experienced the problem/difficulty before while "to know about" only expresses that the speaker has heard or read about it.
Know in (1) refers to the clause that comes right before it, so there's no pronoun necessary -- it's essentially a transform of I know it's your job. In (2), however, the object of know is not indicated, as you point out, so something must be provided.
This depends on what you're actually trying to do. If you simply wish to apply styles to a :before pseudo-element when the a element matches a pseudo-class, you need to write a:hover:before or a:visited:before instead. Notice the pseudo-element comes after the pseudo-class (and in fact, at the very end of the entire selector). Notice also that they are two different things; calling them both ...
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Educational and Fun: The 3d printed wind turbine is ideal for teaching about renewable energy while building a cool, functional gadget. If you’re looking for cool things for 3D printing or want to create an impressive, sustainable project, this wind turbine design is a very good choice.