[css-transitions-1][editorial] Add the word gradual to clarify that it's happening over time

Author: fantasai

css-transitions-1/Overview.bs

@@ -141,7 +141,7 @@ Value Definitions {#values}
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       <p>
-        Normally when the value of a CSS property changes, the rendered result is instantly updated, with the affected elements immediately changing from the old property value to the new property value. This section describes a way to specify transitions using new CSS properties. These properties are used to animate smoothly from the old state to the new state over time.
+        Normally when the value of a CSS property changes, the rendered result is instantly updated, with the affected elements immediately changing from the old property value to the new property value. This section describes a way to specify gradual transitions using new CSS properties. These properties are used to animate smoothly from the old state to the new state over time.
       </p>
       <p>
         For example, suppose that transitions of one second have been defined on the 'left' and