add blendmode support proposal doc

Author: softmarshmallow

docs/css-blend-mode.md

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+# CSS - Blend mode
+
+## Supported types
+
+```
+normal | multiply | screen | overlay | darken | lighten | color-dodge | color-burn | hard-light | soft-light | difference | exclusion | hue | saturation | color | luminosity
+```
+
+## Warning Lack of browser support on SVG element.
+
+Applying `mix-blend-mode` to SVG element is limited in some browsers (safari & mobile browsers) See - https://developer.mozilla.org/en-US/docs/Web/CSS/mix-blend-mode#browser_compatibility
+
+### References
+
+- [`mix-blend-mode`](https://developer.mozilla.org/en-US/docs/Web/CSS/mix-blend-mode)
+- [`background-blend-mode`](https://developer.mozilla.org/en-US/docs/Web/CSS/background-blend-mode)