The main idea to keep in mind is that any tags you use in your html file should be structural in nature: <h1-6> for headers, <p> for paragraphs, <div> for non-paragraphs or other structural divisions, <blockquote> around block quotes, and <em> or <strong> to identify words or phrases that should be emphasized or prominent. Keep 99.99% all
styling in your style sheet.
Devices/apps usually have a default styling, but as mentioned above, it will not look good. You should always tell the device how you want the book to look - even if some devices ignore your wishes.
The way you tell it how you want it to look is using the css stylesheet. You link the stylesheet to your html file by including:
<link href="../Styles/styles.css" type="text/css" rel="stylesheet"/>
in the head of the html file...just replace "../Styles/styles.css" with the path and file name of your css file.
Here is a great resource to help teach yourself (or just reference) all the things you can do with CSS:
W3Schools
Cheers,