It would be great if IkiWiki could apply some sort of preprocessing to
CSS. I'm just "thinking out loud" at the moment, but I might write
a plugin.

The simplest starting point would be concatenating a list of files:

* The actiontabs and monochrome themes are not ready for use as-is;
  the Makefile constructs the real stylesheet by concatenating the
  anti-theme's CSS and the relevant theme's CSS. It would be better
  if they could just drop a file in an underlay, and IkiWiki would
  concatenate the anti-theme stylesheet and the theme's file.

* The blueview theme is the same, but the theme's CSS also starts
  with a couple of stylesheets from YUI. IkiWiki could maybe
  concatenate the anti-theme stylesheet, the two YUI stylesheets
  and the blueview-specific part? Or maybe that's too complicated.

* The goldtype theme is the blueview theme with some overrides
  added to the end. Again, IkiWiki could concatenate all the pieces.

* The [[plugins/contrib/album]] plugin needs to append some
  stuff to the stylesheet, making its installation more involved
  than it ought to be. If it could append the pieces by putting them
  in an underlay, installation would just be a matter of "add
  the files".

I'm not sure whether local.css should be concatenated too, or whether
it should be separate.

It would also be great if the same mechanism could be extended
to compile CSS-like languages like [[!debpkg ruby-sass desc=SASS]]
or [[!debpkg node-less desc=LESS]] down to CSS, but for dependency
reasons, I don't think the themes shipped with IkiWiki should rely on that.

If the compiled CSS ended up with a content-based filename (perhaps
ikiwiki/compiled-HASH.css where HASH is the (possibly truncated) MD5 or SHA1
hash of the content), that would mitigate stale CSS being served from cache
(as long as the HTML has a short expiry, which is desirable anyway),
and ikiwiki-hosting could maybe even do something like this to allow
long-term caching of the files with content-based names:

    <LocationMatch /ikiwiki/compiled-[a-f0-9]+\.css>
        ExpiresByType text/css "now plus 1 year"
    </LocationMatch>

A similar mechanism could maybe be used to minify JavaScript.

## vague syntax proposal: controlled by directive

    \[[!css files="""
        style.css
        ikiwiki/plugin*.css
        ikiwiki/theme*.css
        local.css
    """]]

* *css* directive, placed in any page, concatenates the given files
  and uses them as the stylesheet for that page and its subpages,
  replacing `<TMPL_VAR BASEURL>style.css`

* the files can be globs, which are sorted lexicographically within
  a glob, and do not have to match anything (so it's OK if you don't
  have anything that matches `plugin*.css`); "-" happens to sort
  before ".", so theme-base.css would sort before theme.css,
  which is nice to have

* the default would be `style.css`, then `ikiwiki/plugin*.css`,
  then `ikiwiki/theme*.css` and maybe `local.css`, as in the
  example above

* `style.css` would continue to be the anti-theme

* themes would ship `ikiwiki/theme.css` instead of `style.css`,
  resulting in concatenation

* goldtype would ship `ikiwiki/theme-base.css` (which is a copy of
  blueview, or a symlink to blueview if we can make symlinks safe)
  and `ikiwiki/theme.css` (which is the goldtype additions)

* [[plugins/contrib/album]] would put its templates and
  `ikiwiki/plugin-album.css` in an underlay

* any non-contrib plugin with complicated CSS requirements
  could also move its CSS to an underlay

I think this could be done entirely in a plugin, except for this
change to `page.tmpl` to allow the `<style>`s to be overridden:

    <TMPL_IF COMPILED_CSS>
        <style type="text/css" href="<TMPL_VAR BASEURL><TMPL_VAR COMPILED_CSS>" />
    <TMPL_ELSE>
        <!-- ... what it does now ... -->
    </TMPL_IF>

The plugin could also optionally minify its output, and either pass input
files through an external SASS/SCSS/LESS implementation according
to their extension, or have a hook system to do so.

People who want SASS/LESS would probably just do this in their top-level page:

    \[[!css files="mywiki.less"]]

and do the inclusion/concatenation logic via @import.

Security consideration: the SASS/LESS compilers will read arbitrary local
files, so if you use those languages, ability to upload the appropriate
extension would have to be locked-down. Perhaps it's better to implement
this without that feature initially.

--[[smcv]]

> Although I understand the need to improve CSS inclusion, I wonder why you are
> proposing concatenating CSS rather than including them as several `<link
> type="text/css" href="FILE.css" rel="stylesheet">` lines
> in the header: unless I am missing something, I see this as far more simpler
> than concatenating them.
>
> This would imply that a template variable `CSS` is added to the page
> template, to be filled with those lines.
>
> Whatever solution is used, I agree that such a thing would be useful:
> adding CSS (rather than replacing the existing one) should be easier.
>
> -- [[Louis|spalax]]

>> One big request is more efficient than lots of small requests,
>> if we model the CSS as all changing equally infrequently.
>> In terms of bytes, each file needs some code in the HTML `<head>`,
>> plus the HTTP request and response headers, plus the actual file.
>> On the first page-view, a visitor will have to download all the CSS anyway
>> (one request/response pair per CSS file). On subsequent page-views, there
>> will be one request/"304 Not Modified" response per CSS file, unless the
>> CSS files can be marked "to be cached forever" (which can be done if
>> they have content-based filenames).
>>
>> In terms of time, [[!wikipedia HTTP_pipelining desc="according to Wikipedia"]]
>> browsers don't generally pipeline requests, so the page won't finish
>> loading until one round-trip time per uncached CSS file has elapsed.
>>
>> Having lots of small files with content-based filenames would be the
>> next best thing - not particularly efficient on a generic web server,
>> but they could at least be marked as "cache forever" in server
>> configuration. I'd be OK with doing that if it makes ikiwiki more
>> maintainable, but I don't think concatenating all the CSS at
>> compile time is actually going to be a problem in practice.
>> The individual small files are still going to be available
>> for the wiki operator to edit.
>>
>> If some CSS files change with a significantly different frequency,
>> *then* it might become worthwhile to separate them, but I don't
>> think that's the case (apart from possibly local.css, which is why
>> I'm not sure whether to include it in this).
>> --smcv

>>> I must admit that I am not aware of how those several CSS inclusion lines
>>> tend to make browsing less smooth. Please withdraw my comment.
>>>
>>> As you pointed out, CSS inclusion is more painful than it should be, and
>>> your proposal seems to answer that. Go ahead! --[[Louis|spalax]]