CSS3 browser compatibility question

Hi everyone, 

Once again I feel I have to start by apologising for not posting on here more regularly of late - this year has been a very hectic and busy one, including a change of computer (from Windows XP to Mac OS X) and a number of web design projects!  I hope everyone is keeping well and (at the time of writing) having a Happy Easter!  

As a test to myself and my design abilities I would like to have a go at building a website (or page of a website) using some basic CSS3 properties, such as adding a drop shadow effect to a div.  However, I'm aware that not all browsers support CSS3...particularly Internet Explorer, which tends to be the bane of every web designer's life!  ;)  

What I'd like to try and find out is, what happens when a browser that doesn't support CSS3 tries to read a document that's been styled using CSS3 properties, such as the drop shadow effect, rounded corners, colour gradients, etc.?  Does the page simply display without those elements being visible, i.e., does it just display the div without the drop shadow, rounded corners and with a solid colour, or does something more dramatic happen?  

I look forward to the day when CSS3 and indeed HTML5 are supported by a greater cross-selection of browsers, as I feel that this will add so much more usability, functionality and (dare I use the word) beauty to websites...but I guess that's a whole other discussion!  

If somebody here could help me out with the answer to this question I'd really appreciate it - as always, I'm very grateful to the experts here at Compila Exchange for their superb knowledge.  Many thanks!  

Best wishes, and thanks again,

Chris