highlight active section

[FND]

while it's excellent that we have granular addressability (i.e. anchors) for individual guidelines, when visiting a URL like http://roca-style.org/#unobtrusive-javascript it's often not immediately clear where on the page the respective item resides - this is due to the lack of vertical scrolling (at least on reasonably large screens)

various sites on the web (though the only examples I can think of right now are GitHub's line numbers and @nytimes's Emphasis) highlight the respective section - we should do the same