add proposal docs

Author: softmarshmallow

docs/css-multiple-background.md

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+---
+title: "CSS How to handle multiple background fills"
+version: 0.1.0
+revision: 1
+---
+
+# How to handle multiple background fills
+
+## Definition of `"multiple background fills"`
+
+- one or none active solid fill
+- one or more gradient fill above solid fill
+- one or more image fill
+
+## Possible combinations
+
+single solid fill
+
+```css
+._1 {
+  background: #fff;
+}
+._2 {
+  background-color: #fff;
+}
+```
+
+single solid fill with single gradient fill
+
+```css
+._1 {
+  background-color: #fff;
+  background-image: linear-gradient(to bottom, #fff, #fff);
+}
+
+._2 {
+  background: #fff;
+  background-image: linear-gradient(to bottom, #fff, #fff);
+}
+```
+
+no solid fill with single gradient fill
+
+```css
+._1 {
+  background: linear-gradient(to bottom, #fff, #fff);
+}
+
+._2 {
+  background-image: linear-gradient(to bottom, #fff, #fff);
+}
+```
+
+no solid fill with multiple gradient fill
+
+```css
+._1 {
+  background: linear-gradient(to bottom, #fff, #fff), linear-gradient(to bottom, #fff, #fff);
+}
+```

docs/css-text-gradient.md

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+---
+title: "CSS Gradient on text layer"
+version: 0.1.0
+revision: 1
+---
+
+# CSS - Gradient on Text Layer
+
+Applying a gradient to a text fill is quite different from simply giving a color to a text.
+Yet hooray CSS, it is much more simple than other platforms (flutter, android, ...)
+
+**How to**
+
+```css
+h1 {
+  font-size: 72px;
+  background: -webkit-linear-gradient(#eee, #333);
+  -webkit-background-clip: text;
+  -webkit-text-fill-color: transparent;
+}
+```
+
+### References
+
+- https://cssgradient.io/blog/css-gradient-text/
+- https://github.com/gridaco/designto-code/issues/84

docs/css-text-vertical-align.md

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+---
+title: "CSS Vertically align text content"
+version: 0.1.0
+revision: 1
+---
+
+# Vertical align of text content
+
+### References
+
+- https://stackoverflow.com/questions/8865458/how-do-i-vertically-center-text-with-css