Did you check what content type the server sends?

You see the text instead of rendered HTML content if the server sends the content as text/html. If Firefox sees text/plain then Firefox will show the content as text. You can check that in "Tools > Page Info > General" and with the Live Http Headers extension.

• https://addons.mozilla.org/firefox/addon/live-http-headers/

On Windows it is always possible that security software is interfering and wants to protect you because of an uncommon port number and changes the content type in the HTTP response headers.

When you say "displayed as text" do you mean you see raw HTML source code, or that the CSS is not being applied? I'm assuming the latter.

I suggest reviewing the URLs Firefox is retrieving using the Network Monitor. Open the Web Console using Ctrl+Shift+k, the click the Network tab on the top bar of the console, then reload the page bypassing the cache using Ctrl+Shift+r. Firefox should list the URLs retrieved in attempting to load the page, along with status codes and file sizes, etc.

Anything strange about the CSS files?

If this is an HTTPS site, could you check the URLs for any that might not be loading off the same port (e.g., absolute URL to the host without a port number) in case that might be causing a "cross-site" security issue?

Many thanks for your help, I checked "Page Info" for all three frames consisting the web page. All three of them show "text/html". I saw something interesting - the render mode was set to "Quirks mode". I did some reading about and added "!DOCTYPE" to my tags. The problem still exists - I see in two of the frames HTML source code instead of the rendered HTML. Thanks again, Gidi

I compare the data sent and received by the web-server, I found that the big difference is after an html page is sent to the browser with tag <link rel='stylesheet' type='text/css' href='style.css'>. When the port is 80, the next request from the browser is: received: GET /style.css HTTP/1.1 while in port 8000 it is: received: GET / HTTP/1.1

I tried setting instead of href='style.css' => href='./style.css' but it did not help.

gidigal said

When the port is 80, the next request from the browser is:
received: GET /style.css HTTP/1.1
while in port 8000 it is:
received: GET / HTTP/1.1

That's strange. Firefox definitely should not be confused about a relative URL to a file in the same folder.

Apologies for directing you to a second tool, but could you also try the browser console, where error messages may appear? You can launch that using Ctrl+Shift+j (or the Developer menu), then click Clear at the top to remove existing content. Leave this window open, reload your frameset, then check back in the console to see whether there is any indication of an error parsing the page or a security issue.

I checked browser console, the following error message is displayed twice:

The character encoding of a framed document was not declared. The document may appear different if viewed without the document framing it.

It is displayed for the two frames that show their HTML source code instead of being rendered. Both of these frames contain "<meta http-equiv="Content-Type" content="text/html;charset=UTF-8">" in their source code.

If you right-click the unstyled page and View Page Info, I suspect it will say

Text-Encoding: Windows-1252

or similar.

If you open the CSS file in a tab and check its Text Encoding the same way, what does that one say?

My guess would be the same, but is there a severe mismatch (such as UTF-16LE)?

For main page: Type: text/html Render Mode: Standards compliance mode Encoding: UTF-8

For CSS page: Type: text/css Render Mode: Quirks mode Encoding: windows-1252

I don't think there is a character set incompatibility between UTF-8 and windows-1252; that combination is very common.

I think the problem is related to your earlier note that the wrong URL is requested for the style sheet, and I don't have an explanation for that.

You are sure that there aren't any extended (0x80-0xff) characters in the CSS file, possibly in a comment?

I compared again the GET requests the browser sends to the web server: the first one is "GET / HTTP/1.1", the only difference between port 80 scenario and port 8000 scenario is "Host: localhost" vs "Host: localhost:8000" in the HTTP header. The second request is "GET /style.css HTTP/1.1" (this is a previous request to the request I mentioned before where there's a big difference between port 80 and port 8000). Here, apart from the understood differences of "Host: localhost" vs "Host: localhost:8000" and "Referer: http://localhost/" vs "Referer: http://localhost:8000/" there's a single line existing in port 80 scenario and not in port 8000 scenario. This line contains "live" string and it comes after "Connection: keep-alive" line that exists in both scenarios.