Are you using paragraph and/or character styles in the InDesign document? The CSS file will be for those. If you're only using local overrides, the CSS tags will be inline inside the HTML file, without an external CSS file.
I've got 4 blocks with text inside and a youtube video.
Do they work in the HTML?
You'll only get a CSS file if you have styles defined on the Paragraph Styles panel (Window > Styles > Paragraph Styles). Otherwise, the CSS is inside the HTML file itself.