KMail adds a blue box/button around links. These links are often white or blue. As a result the links are hardly visible. Also no modification should be done to the content of the mail. Please see the attached images. Reproducible: Always I am using Breeze color theme. I am not using custom colors in Kontact/Kmail. The attached images are from Kmail and Zoho webclient.
Created attachment 97666 [details] wordpress zoho
Created attachment 97667 [details] wordpress kontact
Created attachment 97668 [details] getsmartapp zoho
Created attachment 97669 [details] getsmartapp kontact
Created attachment 97966 [details] Discourse forum email example
This is still the case with kmail2 5.3.3 (QtWebEngine), on ArchLinux.