Elixir HD Light strings are made the same as other elixirs, but the gauge of the set is unique. Basically they married medium gauge treble strings with light gauge bass strings to find the optimal string gauge for small bodied Taylor guitars (GA/xx4 and GC/xx2).
The idea behind HD Lights is that both string tension and EQ would be more balanced across the strings, resulting in better playability and better tone.
On the high end, the thicker gauge treble strings give you more body, better definition, and improved action. The lighter bass strings keep the bass clear – it’s not muddy even when you tune down. Along with the balanced EQ and improved playability, you also get a more even volume resulting in easier to control dynamics.
Andy Powers at Taylor came up with the idea for these after redeveloping the 800 series, and these strings are indeed great with the revoiced 600, 800, and 900 series GC and GA guitars. The revoiced models have a stronger, more focused bass in particular, so using lighter bass strings helps keep the low end form being overpowering or becoming muddy. The revoiced models also have a livelier top – so the lighter bass strings still have enough presence without being boomy, and the thicker trebles still have snap, but without being too thin.
Overall, their more balanced EQ coupled with more balanced string tension make HD Lights especially conducive to alternate tunings and ideal for fingerstyle guitar.

Other notes:

Phosphor Bronze vs 80/20 Bronze:
⁃ 80/20 Bronze will be brighter with more zing.
⁃ Phosphor Bronze will be a little warmer with more heft or body. I prefer them for fingerstyle because of their combination of warmth, clarity, and detail.

Nanoweb vs Polyweb: HD Lights are only available in Nanoweb (as of now), which works fine for me, but prior to using this gauge I used Elixir’s Polyweb strings. Though they have slightly different characteristics, I have found both Nanoweb and Polyweb coatings to produce good sounding strings.