Conditional comments not supported since IE10 https://learn.microsoft.com/en-us/previous-versions/windows/internet-explorer/ie-developer/compatibility/hh801214(v=vs.85)?redirectedfrom=MSDN

Vaadin supports only IE11

https://vaadin.com/download/release/8.15/8.15.0/release-notes.html

Conditional comments could be persisted now and page could be requested by old/unsupported browser (IE10-). Hence we need additional check for if/endif in the comment in order to not persist malicous code.