I have the same problem on a freshly installed Kmail/identity. The HTML signature seems just fine in the identity’s configuration page, but when sending an email using it, most of the style is gone and table borders are HTML defaults (wide, grey, ugly).
I found traces of this problem dating years back, here for example : forums.opensuse.org/t/how-to-correctly-style-an-email-signature-in-kmail/133755
Is there something I’m missing for this to work as intended, or is this really a long-standing bug ?
I have the same issue - nothing seems to have changed. Is there any documentation as to what HTML and CSS styling is supported? I have noted that the HTML gets rewritten on save (table bordering gets reset, style=“background-image: url(…);” gets turned into a background= attribute, etc). Starting to think I need to hand code a filter to programmatically add the signature.