@dleon MKV aka Matroska Video is a Container format that comes with different extensions like e.g. WEBM etc., which are all based on the same format. Some of the others only support subgroups of what is allowed inside but definitely everything video fits into it and is streamable if the codec used is fit for it, see among others VP8 or 9 which are used e.g. by Youtube and were especially developed with streaming in mind.
@mozzer A Vivaldi team member stated it is not a bug.
This is a problem of missing font declaration of stylesheet in the page.
Open Settings → Website
Goto section Fonts
Set Standard to Arial or something else
Regarding the HTML of the site.
I inspected the head.
<link href="https://fonts.googleapis.com/css?family=Open+Sans&amp;subset=latin-ext" rel="stylesheet"></head>
If i open Developer Tools and add a type="text/css" to the font declaration, all fonts show up correctly.