Update: The issue appears to have been resolved after upgrading to iOS 11.3. It was observed in iOS 11.2.
When browsing the Web on my iPhone X with Safari, when ever I go to one particular site I occasionally see what appears to be a Face ID validation scan being reported. For e.g., when I land on the website, I see the Face ID overlay superimposed with a spinning and rotating circle, and after a short time the Face ID happy face icon (the one which appears upon successful validation). It disappears a few moments later.
I have occasionally observed this on Slashdot website. It appears every time Safari is relaunched and the aforementioned site is opened. I have not seen this happening on any of the other websites that I browse on my iPhone. However, as I only regularly browse a handful of websites on my iPhone, there may be some reporting bias here.
I don't think this is an HTTPS issue as all the major websites that I visit run over HTTPS.
So, what is special about Slashdot and Face ID? Or is it something set up by Slashdot?