Being able to detect device state at any given moment is important for any number of reasons and so it's important that web app CSS and JavaScript are in sync with each other. In working on the Mozilla Developer Networks' redesign, I found that our many media queries, although helpful, sometimes left JavaScript in the dark about the device state. Is the user viewing the site in desktop, tablet, or phone screen size? Easy from a CSS perspective but CSS doesn't directly speak with JavaScript. I've created a system based on media queries and z-index which can tell me which media query the user is viewing the site in at any given time, so that I can make adjustments to dynamic functionality whenever I want!
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