[gbm]

Originally Posted by iliakan

I added the "color" property to the style of the original HTML-document above:

.link {
    fill: none;
    stroke: #BEC3C7;
    stroke-width: 1px;
    color: red;
  }

Again, the conversion does not keep this .link rule. The "color" property really must be known.
So the "strange properties" is probably not the issue.

I opened the hand-fixed http://ilyakantor.ru/b.epub.zip in Calibre, iBooks, Sigil, all 3 programs programs show it well.

That's the screenshot from Sigil: http://ilyakantor.ru/screen/2015-01-24_0054.png (the path is styled correctly).

So "unsupported properties" is not an issue too here, just because all 3 readers are fine with it.

What remains is a converter issue.

Maybe that's because the converter adds svg: namespace to the svg tag and inside it, and things become messed up?

Running your css though the CSS Validation Service shows these errors:

Spoiler:

bernie