Capability: Visual blending of layers, tiles and/or vector features

[Malvoz]

A (Custom Styling) Capability to enable blending (visually mixing, or "merging") of layers, tiles and/or vector features.

Some of the reference tools have options to change opacity (of tiles, layers, etc.):

• https://docs.microsoft.com/en-us/bingmaps/v8-web-control/map-control-api/tilelayeroptions-object
• https://developers.google.com/maps/documentation/javascript/reference/image-overlay#ImageMapType.setOpacity
• https://openlayers.org/en/latest/apidoc/module-ol_layer_Layer-Layer.html#setOpacity
• https://leafletjs.com/reference-1.7.1.html#imageoverlay-setopacity
• https://docs.mapbox.com/mapbox-gl-js/style-spec/layers/#paint-raster-raster-opacity

However, reducing opacity is problematic because it causes layers, vector features, and (in case of image tiles) text labels to essentially be less visible. Conforming to WCAG success criteria for contrast (e.g. 1.4.3 Contrast (Minimum), 1.4.6 Contrast (Enhanced), 1.4.11 Non-text Contrast) can be hard or even impossible depending on the applied opacity.

mix-blend-mode (and related properties such as background-blend-mode and isolation) may be a better option than opacity in some ways, but - to my understanding - relies on elements belonging to different branches of the DOM tree to blend, which is a necessary consideration (perhaps a limitation).

This capability may be required for the use case: Combine multiple layers of map tile data or features.