I cannot fullscreen in Battle Pirates when playing the HTML5 version. I used to be able to, until about 3 months ago.
The developer puts this on the shoulders of AG, and I believe that may be the case: developer console indicates that Fullscreen is disabled on the Armorgames side (it says "disabled by feature console" ). It does not work in any browser or on any device, in either ChromeOS, macOS, or Windows (including WINE).
I can fullscreen the game in HTML5 when playing my alt account on FB, and when I copy and paste the game address and token that appears in the HTML for the page. This is significant because I am no longer playing out of the AG platform.