... without an associated (X) HTML file?
If what you need to do is CSS related without knowing its associated structure (X) of HTML documents or classes or identifiers ... then it’s just not enough to just write a small page and view it in a browser
Not to offend, but personally I think that previewing CSS without an (X) HTML document is a bit strange. A few reasons:
If you start with CSS, you can try to make your HTML code your CSS when it's the other way around. Starting with CSS and then writing your page down, you feel like a good way to end up with messy HTML, and if I need to choose between (messy HTML and plain CSS) or (plain HTML and messy CSS), I personally would choose blank HTML code every time.
You also specify the <div> and <span> targeting. This is a personal opinion, but I’m very sure that people should not aim at <div> and <span> without class or identifier attributes. Since <div> and <span> have no semantic meaning, I just can’t imagine why you would need to style them if you didn’t reset or comply with the CSS requirement and your client is more likely to ask “all employee names should be underlined "or" spaces should be underlined "? ... but as soon as you target the class and id, you need your HTML document to work with your CSS.