Ethmoid may not completely understand your desire. You have obvously stated you know how to format text so I have no idea why suggestion to format text was provided. Ha ha comprehension of posts is seldom present in these forums and responders generally echo what was originally stated but then it's iLife can't expect much huh?
There is no way to set a universal style to the format of text such as hi, h2 etc. the cumbersome process of formatting text individually via iWeb inspector for the highlighted/selected text is the only avenue available in iWeb. You can not set universal css styles like other, more practical web development tools that are available. So 15 pages with same font style set and published but now you want to change current size from 12 to 15px you would have to go into every page, highligh font, change style via inspector, go to next page repeat process, save and upload all page files. If it were all in one css you could change size in the one css file and upload to see reflected changes on all pages that have set style but iWeb does not work that way; that would be
too easy ha ha 😉
iweb's simplicity is compromised when methods for universal control are convoluted. Meaning you can have "oh so easy drag/drop copy/paste" and not know much, if anything about html, css, etc. but when you go looking for things that utilize technologies not directly available in iWeb you have to deal with how iWeb works or find alternative methods. In this case I suggest the latter option