There is a <style> tag. It's at the end of very long line 9: ...haW4iOnRydWV9" /><style>html,body{height: 100%;}</style><style id="react-native-stylesheet">

There is a <style> tag. It's at the end of very long line 9: ...haW4iOnRydWV9" /><style>html,body{height: 100%;}</style><style id="react-native-stylesheet">[stylesheet-group="0"]{} Several of the first 10 lines are extremely long and have a large number of elements per line. Because it's not meant to be human-readable, to save space, newlines are omitted where they are not required. <head> is optional in HTML5 syntax. It's implied by the first <meta> tag. There's still a <head> element, which you can see in the DOM inspector. <title> is optional. But in Twitter's case, the <title> element appears to be added later by JavaScript, as it's not in the page source but it is in the DOM after loading. Various people say <head> is omitted to save space because it's optional in the HTML5 syntax anyway, and that shaving off a few bytes is worth it. It is to save space, but it seems hardly worth it when you look at the full head section, which has a large number of <link> elements>, and the long content of the origin-trial <meta> elements. They suggest the page is not particularly optimised for size. To see Twitter's head section properly, it's best to view source, copy to a text editor, then insert a newline before each "<" where there isn't one already.