CSS Variables cannot be used from Elm at the moment, `style "--myvar" "value"` is not applied to DOM. Elm version: 0.19 Browsers: All Here are some SSCEs, if CSS variables were applied you would see a 200x200 blue box in this Ellie: https://ellie-app.com/3fpSpnv8nyNa1 Instead, the box is empty  Vanilla HTML + CSS example https://codepen.io/hpate-omicron/pen/BOZzgZ  caniuse data: https://caniuse.com/#feat=css-variables W3 spec: https://www.w3.org/TR/css-variables/