In Sciter <frame> element may have content-style attribute defined.

That content-style attribute takes URL of style sheet that gets applied on top of existing styles of the document:

<frame src="some-child.htm" content-style="content.css" />

That content.css gets appended to the list of other styles of the some-child.htm as if that document has <link rel="stylesheet" type="text/css" href="content.css"> at the end:

    ... local styles ...
   ... local content ...
 <link rel="stylesheet" type="text/css" href="content.css">

So when host document wants:

  1. to customize rendering of child document in the frame
  2. and/or inject some scripting behaviors in it

it can provide its own content.css file without the need of that child element modification.

Sciter SDK built-in document browser is an example of content-style usage:

{sciter-sdk]/doc/main.htm defines content frame as:

<frame #content src="content/preface.htm" content-style="styles/content.css"/>

so each document loaded into the frame gets {sciter-sdk}/doc/styles/content.css applied to its content