Webkit support for different background position keywords

src/css/property-descriptors/background-position.ts

@@ -12,8 +12,6 @@ export const backgroundPosition: IPropertyListDescriptor<BackgroundPosition> = {
     type: PropertyDescriptorParsingType.LIST,
     prefix: false,
     parse: (_context: Context, tokens: CSSValue[]): BackgroundPosition => {
-        return parseFunctionArgs(tokens)
-            .map((values: CSSValue[]) => values.filter(isLengthPercentage))
-            .map(parseLengthPercentageTuple);
+        return parseFunctionArgs(tokens);
     }
 };

src/css/types/length-percentage.ts

@@ -31,14 +31,105 @@ export const getAbsoluteValueForTuple = (
     width: number,
     height: number
 ): [number, number] => {
-    const [x, y] = tuple;
-    return [getAbsoluteValue(x, width), getAbsoluteValue(typeof y !== 'undefined' ? y : x, height)];
+    let [x, y] = tuple;
+    let absoluteX = getAbsoluteValue(x, width);
+    let absoluteY = getAbsoluteValue(typeof y !== 'undefined' ? y : x, height);
+
+    // https://developer.mozilla.org/en-US/docs/Web/CSS/background-position#values
+    // With 3-value syntax, there are two keywords, and an offset that modifies the
+    // preceding keyword
+    if (tuple.length == 3) {
+        // If the first keyword is "left" or "right", we start with the X position
+        if (tuple[0].value == 'left' || tuple[0].value == 'right') {
+            // If the second tuple is an offset, apply it to the X position
+            if (tuple[1].type == TokenType.PERCENTAGE_TOKEN || tuple[1].type == TokenType.DIMENSION_TOKEN) {
+                if (tuple[0].value == 'left') absoluteX = getAbsoluteValue(tuple[1], width);
+                if (tuple[0].value == 'right') absoluteX = width - getAbsoluteValue(tuple[1], width);
+            }
+
+            // If the second tuple is a keyword, X position is left or right
+            if (tuple[1].type == TokenType.IDENT_TOKEN) {
+                if (tuple[0].value == 'left') absoluteX = getAbsoluteValue(tuple[0], width);
+                if (tuple[0].value == 'right') absoluteX = width - getAbsoluteValue(tuple[0], width);
+            }
+
+            // If the first keyword is left/right and the last is 50%, we know that's
+            // vertical centering
+            if (tuple[2].type == TokenType.PERCENTAGE_TOKEN) {
+                absoluteY = getAbsoluteValue(tuple[2], height);
+            }
+        }
+
+        // If the third tuple is an offset (meaning the first two are keywords) it modifies
+        // the second tuple keyword
+        if (tuple[2].type == TokenType.PERCENTAGE_TOKEN || tuple[2].type == TokenType.DIMENSION_TOKEN) {
+            if (tuple[1].value == 'top') absoluteY = getAbsoluteValue(tuple[2], height);
+            if (tuple[1].value == 'bottom') absoluteY = height - getAbsoluteValue(tuple[2], height);
+            if (tuple[1].value == 'left') absoluteX = getAbsoluteValue(tuple[2], width);
+            if (tuple[1].value == 'right') absoluteX = width - getAbsoluteValue(tuple[2], width);
+        }
+
+        // If the first keyword is "center" we check to see if the other keyword is left/right
+        // to see if this applies to Y, otherwise assume X
+        if (tuple[0].value == 'center') {
+            if (
+                tuple[1].value !== 'left' &&
+                tuple[1].value !== 'right' &&
+                tuple[2].value !== 'left' &&
+                tuple[2].value !== 'right'
+            ) {
+                absoluteX = getAbsoluteValue(tuple[0], width);
+            }
+
+            if (tuple[1].value == 'left' || tuple[1].value == 'right') {
+                absoluteY = getAbsoluteValue(tuple[0], height);
+            }
+        }
+
+        if (tuple[1].value == 'center' || tuple[2].value == 'center') {
+            if (tuple[0].value == 'right' || tuple[0].value == 'right') {
+                absoluteY = getAbsoluteValue(tuple[2], height);
+            }
+        }
+    }
+
+    // With 4-value syntax, the 1st and 3rd values are keywords, and the 2nd and 4th
+    // are offsets for each of them respectively
+    if (tuple.length == 4) {
+        if (tuple[0].type == TokenType.IDENT_TOKEN) {
+            if (tuple[0].value == 'left') absoluteX = getAbsoluteValue(tuple[1], width);
+            if (tuple[0].value == 'right') absoluteX = width - getAbsoluteValue(tuple[1], width);
+            if (tuple[0].value == 'top') absoluteY = getAbsoluteValue(tuple[1], height);
+            if (tuple[0].value == 'bottom') absoluteY = height - getAbsoluteValue(tuple[1], height);
+        }
+
+        if (tuple[2].type == TokenType.IDENT_TOKEN) {
+            if (tuple[2].value == 'top') absoluteY = getAbsoluteValue(tuple[3], height);
+            if (tuple[2].value == 'bottom') absoluteY = height - getAbsoluteValue(tuple[3], height);
+            if (tuple[2].value == 'left') absoluteX = getAbsoluteValue(tuple[3], width);
+            if (tuple[2].value == 'right') absoluteX = width - getAbsoluteValue(tuple[3], width);
+        }
+    }
+
+    return [absoluteX, absoluteY];
 };
 export const getAbsoluteValue = (token: LengthPercentage, parent: number): number => {
     if (token.type === TokenType.PERCENTAGE_TOKEN) {
         return (token.number / 100) * parent;
     }
 
+    if (token.type === TokenType.IDENT_TOKEN) {
+        switch (token.value) {
+            case 'right':
+            case 'bottom':
+                return parent;
+            case 'center':
+                return 0.5 * parent;
+            default:
+                return 0;
+        }
+    }
+
     if (isDimensionToken(token)) {
         switch (token.unit) {
             case 'rem':

tests/reftests/background/position.html

@@ -27,7 +27,15 @@
         border:1px solid #000;
       }
 
-      .small, .medium{
+      .medium-wide div{
+        width:200px;
+        height:160px;
+        float:left;
+        margin:10px;
+        border:1px solid #000;
+      }
+
+      .small, .medium, .medium-wide{
         clear:both;
       }
 
@@ -55,6 +63,13 @@
       <div style="background:url(../../assets/image.jpg) no-repeat center center;"></div>
     </div>
 
+    <div class="medium-wide">
+      <div style="background:url(../../assets/image.jpg) no-repeat right 30px bottom 40px;"></div>
+      <div style="background:url(../../assets/image.jpg) no-repeat left bottom -40px;"></div>
+      <div style="background:url(../../assets/image.jpg) no-repeat right 30% top 20%;"></div>
+      <div style="background:url(../../assets/image.jpg) no-repeat center right 20px;"></div>
+    </div>
+
     <div class="medium">
       <div style="background:url(../../assets/image.jpg) -15% 25px;"></div>
       <div style="background:url(../../assets/image.jpg) repeat-x 50px 50px;"></div>