You have now learned enough CSS to make good Web sites.  Unfortunately you will probably have forgotten a lot of what you have learned.  This is not unusual it is the way the human mind works.  For the knowledge to stay in your brain it needs to become more than just knowledge.  You need to do it.  Keep doing it and it will stick and become easy.  There is some practise coming up but as ever make sure you keep trying all this (and more) on your own pages.  Don't mess with the example pages which you have created in intermediatesite but copy them as needed into a new folder and play with them as much as you want.

You might not remember being a young child but if you do you will remember that you learned by trying stuff.  Often you failed but you kept trying and discovered what works and what doesn't.

Standards

Remember that there are rules and standards.  Try to follow what it says here but also check your code on the HTML validator and the CSS validator.  Trust the Worldwide Web Consortium documentation rather than sites such as w3schools.  You may not understand them at first but they are the only ones guaranteed to be correct because they set the standards.

Browsers

Not all browsers fully follow the standards.  Most modern ones at least try to but some older ones deliberately didn't.  If a page does not work try it in a different browser.  If a page does work try it in a different browser!  Your users will be using all sorts of browsers including out of date ones so you need to know what will always work.

Browser defaults

Remember also that just because you have not styled something does not mean that there are no styles.  Each browser has a set of default styles which it will use if you do not include any.  These will be slightly different between browsers so if you want your page to look the same in all browsers make sure you set as many styles as possible and leave nothing to the default styles.  This is not an easy job - search for css browser reset and you will find many different attempts to cover everything which you could adapt for your own use.