fix(css): replace value 'center' with '0% 0%' for mask-position inita…

…l value (#714)

…l value

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### Description

The page says that the mask-position Initial value is center, but when I
tested it in Chrome, Firefox, etc., it was not center.
So I changed the initial value of the mask-position property from center
to 0% 0%.

### Motivation

It will be helpful for people working with CSS by referring to the mdn
document.

### Additional details

#713
https://developer.mozilla.org/en-US/docs/Web/CSS/mask-position

### Related issues and pull requests

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css/properties.json

@@ -6874,7 +6874,7 @@
     "groups": [
       "CSS Masking"
     ],
-    "initial": "center",
+    "initial": "0% 0%",
     "appliesto": "allElementsSVGContainerElements",
     "computed": "consistsOfTwoKeywordsForOriginAndOffsets",
     "order": "perGrammar",