Validate nested HTML-style tags

A tiny templating engine hands you a stream of tags already tokenized for you: an opener is a name like "div", a closer is the same name prefixed with /, like "/div". A document is well formed when every closer matches the most recently opened, still-open tag, and nothing is left open at the end.

["div", "span", "/span", "/div"]   -> True
["div", "/span"]                   -> False   # closes the wrong tag
["p", "b", "/p", "/b"]             -> False   # crossed nesting
["a"]                              -> False   # never closed

Return whether the tag stream is well formed. Aim for $O(n)$.