We are very happy to announce that QGIS 3.4.6 LTR is now OGC certified as a reference implementation : 
The OGC certification program gives a third party validation that the a web service is compliant with the standard.
The certification process requires manual work, so we will only certify on version for each LTR. This was not enough, so we build a OGC CI test platform that is checking compliance every night for WMS and WFS, so that you can check by yourself any specific version commit.
However WMS 1.3 is only the basics, if you rely on other services like WFS, WCS, or advanced capabilities like Raster or Vector Elevation, we are looking for supporters!
As the future is almost now If you want QGIS to be on the cutting edge with the upcoming WFS3, a JSON-REST modern version of WFS, please get in touch. We’d love to push this both into QGIS server and Desktop.