• Sean McGivern's avatar
    Optimise email CSS for speed with Premailer · f4cedacc
    Sean McGivern authored
    Remove all descendant selectors from the push email styling, to
    drastically reduce CPU time when inlining the CSS for syntax-highlighted
    diffs.
    
    Background:
    
    Premailer is a Ruby gem that inlines CSS styles from an external
    stylesheet before emails are sent, so that they are compatible with
    Gmail. At a high level, it parses the CSS files it finds, and parses the
    email body with Nokogiri. It then loops through the selectors in the
    CSS, using Nokogiri to find matching elements, and adds inline
    styles. (It does more than this, like merging styles applied to the same
    element, but that's not relevant to this issue.)
    
    Nokogiri converts CSS selectors to XPath first, like so:
        Nokogiri::CSS.xpath_for('foo bar')
        # => ["//foo//bar"]
    
    On documents with high node counts (say, a syntax-highlighted copy of
    jQuery), having both descendant selectors is very expensive. Both
    `//foo/bar` and `//bar` will be much more efficient, although neither
    are directly equivalent.
    
    An example, on a document containing two syntax-highlighted copies of
    jQuery:
        Benchmark.realtime { p doc.search('.o').count }
        # 9476
        # => 0.3462457580026239
        Benchmark.realtime { p doc.search('.code.white .o').count }
        # 9476
        # => 85.51952634402551
    
    The performance is similar for selectors which _don't_ match any
    elements, and as Premailer loops through all the available selectors, we
    want to avoid all descendant selectors in push emails.
    
    Because of the theming support in the web UI, all syntax highlighting
    selectors are descendant selectors of classes like `.code.white` or
    `.code.monokai`. There are over 60 CSS classes for syntax highlighting
    styles alone, all of which are expressed in the inefficient form above.
    
    In emails we always use the white theme, and were reusing the same CSS
    file. But in emails, we don't need to descend from `.code.white` as that
    will always be the theme, and we can also remove some other selectors
    that are only applicable to the web UI. For the remaining descendant
    selectors, we can convert them to child selectors, type selectors, or
    class selectors as appropriate.
    
    As in the example above, having no descendant selectors at all in the
    push email CSS can provide a drastic (and surprising) performance
    improvement.
    f4cedacc
notify.scss 314 Bytes