If your mother had to buy a HDR TV pre-DisplayHDR, then you'd either have to:
1. Let her rely purely on marketing material
2. Tell her to dig deep into each TVs specifications for a collection of parameters & protocols that might not me prominent, if they're listed at all, and give her an understanding of how to judge those specifications.
Now, you can just say "Make sure it's DisplayHDR certified, the higher the number the brighter, some can also go very dark". Is that not a lot easier?
Sure, DisplayHDR technically doesn't specify *which* HDR protocols a display supports, but given it's a paid-for certification process you'd assume any genuinely HDR display would at least support the open standard used by most media, HDR10(And I doubt any display with at least DisplayHDR500 will ever release without that support).