MRC just revoked the accreditation from AppNexus. According to the Media Rating Council Accreditation Updates, January through March 2018, AppNexus accreditation was "revoked for all previously accredited metrics (Desktop Display Served and Viewable Ad Impressions and related viewability metrics); AppNexus has withdrawn from the accreditation process." AppNexus informed

Viewability metrics are frequently found on the best DSP's, where by default, the DSP is measuring the viewability of all impressions bought. If the DSP does not measure the viewability, you will be able to integrate a third party provider to measure the viewability, like IAS or Meetrics.