Like other web standards, these things tend to get implemented in a piecemeal fashion according to consensus and demand. By the time the standards are published, much of their content is already implemented in modern browsers.
For example, the ES6 spec was just published a few months ago, so it will still be a year before we see ES7. In spite of that, the async / await capabilities are sufficiently compelling and agreed upon that patches are already being reviewed for Firefox: https://bugzilla.mozilla.org/show_bug.cgi?id=1185106
Kangax's tables, linked in another reply, are a great way to check what the support situation is like across browsers.
For example, the ES6 spec was just published a few months ago, so it will still be a year before we see ES7. In spite of that, the async / await capabilities are sufficiently compelling and agreed upon that patches are already being reviewed for Firefox: https://bugzilla.mozilla.org/show_bug.cgi?id=1185106
Kangax's tables, linked in another reply, are a great way to check what the support situation is like across browsers.