What is the difference between `lightgrey` and `lightgray` in CSS?

According to CSS3 Color, there is no difference:

┌────────────┬─────────┬─────────────┐
│ Color name │ Hex rgb │   Decimal   │
├────────────┼─────────┼─────────────┤
│ lightgray  │ #d3d3d3 │ 211,211,211 │
│ lightgrey  │ #d3d3d3 │ 211,211,211 │
└────────────┴─────────┴─────────────┘

en-US:

.foo { color: gray; }

en-UK

.foo { colour: grey; }

since colouris not supported, use gray

They are exactly the same color , check this article for more information.

Gray is the more popular spelling in the US, while grey reigns supreme in the UK.