2018 is a great year for element queries!
Right now there is no spec for CSS, but preliminary work is being done:
Element Queries are listed by the W3 under 'Exploratory Work' for responsive technologies: w3.org/2018/04/web-roadmaps/mobile/adaptation.html
The Web Platform Incubator Community Group (WICG) is working on a "Use Cases and Requirements" document they can use to get the CSS Working Group (CSSWG) and browser makers interested in the idea hopefully to work with them to produce a spec for element queries in CSS: github.com/WICG/cq-usecases
Mozilla has already put Element Queries, and the work the WICG is doing right now on their roadmap for 2018, so it seems like they have committed to review what the WICG comes up with and consider whether they should pursue it or not: github.com/WICG/cq-usecases
So that's where specification is at on the part of W3, WICG, and browser makers, in the meantime there are more plugins and websites using these plugins than ever before too!
It's been possible for 15+ years to use JavaScript plugins to make websites responsive with element queries, but it wasn't until the last ~5 years that people actually started building this way. Now that some people have been building sites like this for a long time (half a decade!) we're starting to see some really great optimizations and plugins that offer more power than the previous generation of plugins.
If you're not using element queries in your designs today, there's nothing to wait for. For now we'll rely on JavaScript plugins to help us achieve them, and hopefully sometime in the future a CSS-native approach may arrive, but that is still years in the future (if it ever happens).