Do not parse caption-side: left / right as valid CSS.

third_party/WebKit/Source/core/css/CSSProperties.json5

 {
 // This file specifies all the CSS properties we support and the necessary
 // information for our code generation. The various supported arguments
 // are described below with example usage
 
   parameters: {
     // - alias_for: "other-property"
     // Properties specifying alias_for should be virtually identical to the
     // properties they alias. Minor parsing differences are allowed as long as
     // the CSSValues created are of the same format of the aliased property.
@@ skipping 825 matching lines @@
     {
       name: "buffered-rendering",
       svg: true,
     },
     {
       name: "caption-side",
       independent: true,
       inherited: true,
       default_value: "top",
       field_template: "keyword",
-      keywords: ["top", "bottom", "left", "right"],
+      keywords: ["top", "bottom"],
     },
     {
       name: "caret-color",
       api_class: true,
       api_methods: ["parseSingleValue"],
       custom_all: true,
       inherited: true,
       interpolable: true,
     },
     {
@@ skipping 2334 matching lines @@
     {
       name: "-webkit-transition-timing-function",
       alias_for: "transition-timing-function",
     },
     {
       name: "-webkit-user-select",
       alias_for: "user-select",
     },
   ],
 }