Is it possible to parse a stylesheet with Nokogiri? - css

Question

I've spent my requisite two hours Googling this, and I can not find any good answers, so let's see if humans can beat Google computers.
I want to parse a stylesheet in Ruby so that I can apply those styles to elements in my document (to make the styles inlined). So, I want to take something like
<style>
.mystyle {
color:white;
}
</style>
And be able to extract it into a Nokogiri object of some sort.
The Nokogiri class "CSS::Parser" (http://nokogiri.rubyforge.org/nokogiri/Nokogiri/CSS/Parser.html) certainly has a promising name, but I can't find any documentation on what it is or how it works, so I have no idea if it can do what I'm after here.
My end goal is to be able to write code something like:
a_web_page = Nokogiri::HTML(html_page_as_string)
parsed_styles = Nokogiri::CSS.parse(html_page_as_string)
parsed_styles.each do |style|
existing_inlined_style = a_web_page.css(style.declaration) || ''
a_web_page.css(style.declaration)['css'] = existing_inlined_style + style.definition
end
Which would extract styles from a stylesheet and add them all as inlined styles to my document.

Answer 1

Nokogiri can't parse CSS stylesheets.
The CSS::Parser that you came across parses CSS expressions. It is used whenever you traverse a HTML tree by CSS selectors rather than XPath (this is a cool feature of Nokogiri).
There is a Ruby CSS parser, though. You can use it together with Nokogiri to achieve what you want.
require "nokogiri"
require "css_parser"
html = Nokogiri::HTML(html_string)
css = CssParser::Parser.new
css.add_block!(css_string)
css.each_selector do |selector, declarations, specificity|
html.css(selector).each do |element|
style = element.attributes["style"]&.value || ""
element.set_attribute('style', [style, declarations].compact.join(" "))
end
end

Answer 2

#molf definitely had a great start there, but it still required debugging a handful of problems to get it working in production. Here is the current, tested version of this:
html = Nokogiri::HTML(html_string)
css = CssParser::Parser.new
css.add_block!(html_string) # Warning: This line modifies the string passed into it. In potentially bad ways. Make sure the string has been duped and stored elsewhere before passing this.
css.each_selector do |selector, declarations, specificity|
next unless selector =~ /^[\d\w\s\#\.\-]*$/ # Some of the selectors given by css_parser aren't actually selectors.
begin
elements = html.css(selector)
elements.each do |match|
match["style"] = [match["style"], declarations].compact.join(" ")
end
rescue
logger.info("Couldn't parse selector '#{selector}'")
end
end
html_with_inline_styles = html.to_s