How it works
Your JPEG stays a normal JPEG. For the web we add an ISO 21496-1 gain map — a second, tiny grayscale image that tells HDR-aware software how much brighter each pixel may go. For LinkedIn we write the pixels in BT.2100 PQ with the matching ICC profile, which LinkedIn preserves.
What glows best
Light, near-white elements on dark backgrounds. Strong (+2.9 stops, ~1,500 nits) is the measured real-world sweet spot. Dark colors can't really glow — boosting them reads as washed neon.
Honest limits
HDR only shows on HDR displays in software that honors gain maps or PQ profiles (Chrome, Safari 26, Apple Photos, LinkedIn's apps). Most other platforms strip or normalize it — the file still looks perfectly normal there.