Can you recommend a good CSS online resource or book?
Are there any favorite online references or good introductory and intermediate/advanced books on this topic that you'd recommend?

The backstory is that I'm working on a web development project that is "re-skinning" our existing web site to make it more usable and visually appealing given current technologies that either weren't available or widely adopted as the site evolved (over the last ten-plus years).  Of the new technologies and methods, CSS, although not really all that new, is the most interesting to me because it is clear that it has great power beyond simple styling of fonts and colors - and, as the saying goes, "With great power comes great responsibility."  In other words, it is (now) clear to me that CSS is more than a way to apply a look and feel to page markup but is a way to *apply behaviors* to web pages.  So it seems to me that it will be required to understand at a fundamental level "how CSS works" not so much from a page designer's perspective but from that of a programmer or software engineer in order to find and correct "bugs" in the behaviors of a given web page.

So, I'm looking for a place to start...
Can you recommend a good CSS online resource or book?
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Are there any favorite online references or good introductory and intermediate/advanced books on this topic that you'd recommend?

The backstory is that I'm working on a web development project that is "re-skinning" our existing web site to make it more usable and visually appealing given current technologies that either weren't available or widely adopted as the site evolved (over the last ten-plus years).  Of the new technologies and methods, CSS, although not really all that new, is the most interesting to me because it is clear that it has great power beyond simple styling of fonts and colors - and, as the saying goes, "With great power comes great responsibility."  In other words, it is (now) clear to me that CSS is more than a way to apply a look and feel to page markup but is a way to *apply behaviors* to web pages.  So it seems to me that it will be required to understand at a fundamental level "how CSS works" not so much from a page designer's perspective but from that of a programmer or software engineer in order to find and correct "bugs" in the behaviors of a given web page.

So, I'm looking for a place to start...