Non-existent property "solid-stroke" used

I recently created a new SVG document within Inkscape. When I had saved the file as an optimised SVG file and opened it in Notepad, I found a so-called "solid-stroke" property under an ellipse element's style attribute, followed by a colour value. I tried to find this property in the SVG specifications, but could not find it:

<ellipse style="stroke-linejoin:round;solid-stroke:#00a5ec;stroke-linecap:round;stroke-width:100;fill:none" rx="116.8" ry="116.8" cy="225" cx="225"/>

Is it supposed to be a special Inkscape only property?
Maybe it is supposed to be a "stroke" property value instead...

I think I saw some other properties that also don't exist in the specifications, when I used the XML editor within Inkscape to view said properties with. However, the "solid-stroke" property was the only one that was actually saved.

I'm using Inkscape 0.91 on Windows 10.