Avoid duplicates between color-contrast and color-contrast-enhanced

[WilcoFiers]

Product

axe-core

Feature Description

Currently the color-contrast-enhanced rule fails nodes that also fail the color-contrast rule. This creates duplicates when both rules are on. This isn't ideal. A better solution would be to pass nodes that are below the contrast threshold of color-contrast.

There are already options available in color-contrast-evaluate to do this. We should add caching to this evaluate method to avoid calculating the contrast of a node more than once though, since this is a fairly slow operation.

[WilcoFiers]

Can probably memoize getBackgroundColor, that'll be the quickest way to cache this.

[padmavemulapati]

Validated with the latest develop branch code base,
when run both color-contrast-enhanced and color-contrast , now it is not resulting both incomplete results instead it is giving only color-contrast incomplete result and avoiding duplicate

Test snippet:

<p id="target" style="color: #666; background: linear-gradient(to right, #FFF, #0FF); width: 300px">
    Some text in English
  </p>
  <p id="target" style="color: #666; background: linear-gradient(to right, #FFF, #0FF); width: 300px; font-size: 18pt;">
    Some text in English
  </p>