SharePoint CSS Registration adding "25" to "/Style%2520Library/" for font definitions

Question

We have font definition files that are referenced from a CSS file that is included in our SharePoint Online Master Page. They are .woff, .svg, .avg, .ttf, .otf. The CSS is loaded with CSS Registration like this:
<SharePoint:CssRegistration Name="/Style%20Library/content/app/css/app.css" runat="server"/>

For some reason SharePoint will append a 25 to the Style Library path where we have the fonts stored where the HTTP escape code for space is, turning %20, or space, into %2520. "Style Library" is the only part of the URL with a space in it.

While it should be /Style%20Library/ it's coming as /Style%2520Library/

In the CSS file, which is in a folder under "Style Library", they are referenced like this:

@font-face {
    font-family: "Effra Light";
    src: url(../css/fonts/Effra_Std_Lt.woff), 
         url(../css/fonts/Effra_Std_Lt.svg), 
         url(../css/fonts/Effra_Std_Lt.ttf), 
         url(../css/fonts/Effra_Std_Lt.eot), 
         url(../css/fonts/Effra_Std_Lt.otf);
}

And finally I've noticed that this error does not happen as frequently when directly opening a page via a URL for the first time. It happens more frequently when a Page is re-loaded by SharePoint, like when a User clicks on a List operation or changes the Page to Edit Page mode. Obviously, one of the SharePoint mechanics is altering the URL. How to prevent this?

Perhaps a related ticket here

Answer 1

I had the same problem. In the masterpage, we had this:

<SharePoint:CssRegistration Name="<% $SPUrl:~sitecollection/Style%20Library/Branding/css/Seattle.css %>" runat="server" />

For some reason, SharePoint re codifies the %20 adding %2520 that gives error in some cases not loading the CSS.

To solve this, replace the %20 for a space in the registration, like this:

<SharePoint:CssRegistration Name="<% $SPUrl:~sitecollection/Style Library/Branding/css/Seattle.css %>" runat="server" />

SharePoint will transform Style Library to Style%20Library automatically without the 25, doing the trick and working just fine.