I believe this has been fixed in 1.21.2 now?
Can we close it?

Summary:
stylesheet and icon <link> elements not properly classified as page requisites

The version of GNU Wget I was using:
1.20.3 from https://eternallybored.org/misc/wget/releases/wget-1.20.3-win64.zip
(None of the changes listed in NEWS for newer versions seem relevant to this issue.)

How I invoked wget:

wget --debug --page-requisites https://mdn.github.io/css-examples/alt-style-sheets/

What I expected wget to do:
fetch https://mdn.github.io/css-examples/alt-style-sheets/ and save as index.html
fetch https://mdn.github.io/css-examples/alt-style-sheets/default.css and save as default.css
fetch https://mdn.github.io/css-examples/alt-style-sheets/simple.css and save as simple.css
fetch https://mdn.github.io/css-examples/alt-style-sheets/fancy.css and save as fancy.css

What wget did:
fetch https://mdn.github.io/css-examples/alt-style-sheets/ and save as index.html
fetch https://mdn.github.io/css-examples/alt-style-sheets/default.css and save as default.css

Output messages (Hopefully the relevant portion; the rest can be provided if needed):

[IRI Enqueuing 'https://mdn.github.io/css-examples/alt-style-sheets/default.css' with 'utf-8'
Not following due to 'link inline' flag: https://mdn.github.io/css-examples/alt-style-sheets/simple.css
Not following due to 'link inline' flag: https://mdn.github.io/css-examples/alt-style-sheets/fancy.css
Not following due to 'link inline' flag: https://developer.mozilla.org/en-US/docs/Web/HTML/Element/link

Comments:

The page used in the example above references three stylesheets:

<link href="default.css" rel="stylesheet" type="text/css" title="Default Style">
<link href="simple.css" rel="alternate stylesheet" type="text/css" title="Simple">
<link href="fancy.css" rel="alternate stylesheet" type="text/css" title="Fancy">

Wget only considers the first one (with rel="stylesheet") to be a page requisite as far as --page-requisites is concerned.  This seems incorrect because the value of the rel attribute is specified by the HTML Standard section 4.2.4 (The link element) to be an unordered set of unique space-separated tokens.  All three in this example include "stylesheet", so they should all be considered page requisites.

A similar problem occurs with favicons.  Wget seems to consider those referenced with rel="shortcut icon" to be page requisites, but not those referenced with rel="icon".  For an example of the latter, try the following:

wget --page-requisites https://savannah.gnu.org/

The relevant portion of the HTML is:

<link rel="icon" type="image/png" href="/images/Savannah.theme/icon.png" />

It seems that rel="shortcut icon" is actually deprecated but still allowed for historical reasons.  See HTML Standard section 4.6.6.8 (Link type "icon"), especially the note at the end of the section.

Posssible cause and solution:

I haven't investigated thoroughly and I'm not very familiar with C, but I'm guessing that the function tag_handle_link in the file src/html-url.c is responsible for this issue and is just checking the rel attribute of the <link> element for case-insensitive equality to "stylesheet" or "shortcut icon".

A better approach might be to split the rel attribute on ASCII whitespace as described by the HTML Standard section 4.6.6 (Link types) and then check each of the resulting tokens for case-insensitive equality to "stylesheet" and "icon".

An alternative which would probably be less correct and less reliable but may be easier to implement and still useful would be to check the rel attribute for case-insensitive equality to "stylesheet", "alternate stylesheet", "stylesheet alternate", "shortcut icon", and "icon".


See also:
HTML Standard section 4.6.6.1 (Link type "alternate")
HTML Standard section 4.6.6.22 (Link type "stylesheet")
<link>: The External Resource Link element (MDN)
Link types (MDN)