There seems to be a difference in file referecense when using 

   %rest:path("/show/{$welcome}") 
   %method("html")

vs.

   %rest:path("/show")
   %rest:form-param("welcome", "{$welcome}")
   %method("html")

In the first version it appears that references within to folder /static inside the BaseX web directory are not found (images stylesheets etc.), while it works nicely using parameter.

This snippet may illustrate, the secon shows the logo, the other won't:

module namespace page = 'http://basex.org/modules/web-page';

declare 
%rest:path("/show/{$welcome}")
%output:method("html")
function page:show($welcome) {
  <html><body><img src="static/logo.png"/><p>{$welcome}</p></body></html>
};

declare 
%rest:path("/show")
%rest:form-param("welcome", "{$welcome}")
%output:method("html")
function page:showparam($welcome) {
  <html><body><img src="static/logo.png"/><p>{$welcome}</p></body></html>
};