Complex sentence: HDMI 1.2 and all later versions allow any arbitrary resolution and frame rate (within the bandwidth limit); any formats that are not supported by the HDMI Specification (i.e. no standardized timings defined) may be implemented as a vendor-specific format.
Split into simpler sentences: HDMI 1.2 and all later versions allow any arbitrary resolution and frame rate (within the bandwidth limit). Formats the HDMI Specification doesn't support (i.e., no standardized timings defined) may be implemented as a vendor-specific format.