Q. How do you cite the online version of the CRC using the ACS citation style?

You will have to download the pdf of the section of the CRC Handbook that you are looking at. You need to do this in order to identify the section and page number that you are referring to. What you are seeing on the webpage does not provide this information. Because it is in pdf format, you are now technically looking at the print edition. You would cite it like this:

Example:

CRC Handbook of Chemistry and Physics, 84th Ed.; Lide, D.R., Ed.; CRC Press: Boca 
    Raton, FL, 2003; Section 3, p 476.

Do not click the "how to cite" button. This is showing you how to cite the entire section, not the page containing the element you are looking for.