Is there a list of officially supported Macs for OS X El Capitan?
I ask to get ready if any hardware which is supported on Yosemite is not supported by El Capitan?
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If you care to read the tea leaves, the initial OS X v10.11 Developer Beta supports the following Macs:
No official requirements are published by Apple.
About your last question, no. There are no macs supported by Yosemite that are not supported by El Capitan.
It's worth noting that every Mac that supports Mountain Lion (10.8) also supports Mavericks (10.9), Yosemite (10.10), and El Capitan Developer Beta 1 (10.11). System requirements in El Capitan are unlikely to change from those in the developer beta.
However, some features might be unavailable on earlier systems. There is informed speculation that Metal, for example, might only work with newer GPUs.