Practically speaking, if I buy an ISA VGA card, will it implicitly support CGA and EGA games?
All VGAs support the “official” CGA and EGA modes, so most CGA and EGA games work fine. However, compatibility can only be relied upon at the BIOS level; a VGA can be implemented without strict hardware compatibility with the older standards. This affects some CGA games in particular.
There is a CGA test tool available. It was written following this discussion about CGA compatibility. This other discussion summarises test results on a variety of VGAs; basically, older Trident, Paradise and ATI boards provide good CGA compatibility (some with the help of an additional TSR or configuration tool), Tseng boards not so much. Trident’s TSR can cause timing issues with some games; Paradise and ATI boards switch to a hardware-supported CGA compatible mode instead.
Note that these considerations vary to some extent depending on the system you want to use the VGA in: CGA compatibility is only really important in older machines (8088/8086 or 286s), because many of the CGA-specific games which cause issues on EGA or VGA won’t run well on faster machines anyway. An extreme example of this is the 8088 MPH demo, mentioned by traal (and whose authors include the developer of the CGA test tool mentioned above), which uses cycle-counting at all levels (instruction execution, memory timing, and even NTSC output) to achieve its effects — as a result, it only works correctly on an IBM 5150 with CGA and a composite monitor, or computers which are cycle-exact compatibles.
The other answers are good, but one specific area where VGA cards are generally incompatible with CGA is with respect to CGA's Composite mode. In the video "CGA Graphics - Not as bad as you thought!", The 8-Bit Guy demonstrates the CGA Composite mode and explains that, after about 1987 or so, new software written specifically for CGA Composite Mode became uncommon because later cards did not support it.
To be clear, Composite Mode (as opposed to the RGBI mode that most people are familar with) only affects what colors are seen on the screen, it does not affect whether a picture can be made to show up at all.
I have no relationship with The 8-Bit Guy.
Yes, most EGA and CGA games work just fine with a VGA card. VGA isn't 100% backwards compatible with EGA and CGA but in practice I don't think many games tried to get too fancy with these older cards. Gaming on IBM PC compatibles didn't really become a thing until VGA became standard on PCs, before that most IBM PC games were ports from other platforms. A bigger problem with compatibility is CPU speed, a lot of older CGA and EGA games have a fairly narrow range of CPU speeds they work well with.