1. element api: checkvisibility

  2. element api: checkvisibility: `options.checkopacity` parameter

  3. element api: checkvisibility: `options.checkvisibilitycss` parameter

  4. element api: checkvisibility: `options.contentvisibilityauto` parameter

  5. element api: checkvisibility: `options.opacityproperty` parameter

  6. element api: checkvisibility: `options.visibilityproperty` parameter