What is the Indentation standard for HTML (Tab / Two spaces / etc)?

As nobody mentioned it, the Google HTML/CSS Style Guide and the W3School HTML(5) Style Guide recommend 2 spaces.

This article also brings an analysis of the effect of tabs vs spaces in the resulting file size.

Do not use tabs; use two spaces. Tabs are problematic because they can be a different width in different editors (and terminals, for that matter). Since whitespace is removed when you tidy your HTML before deployment, there is no ultimate bandwidth benefit to using tabs; they simply introduce a wildcard into your development experience.

Two spaces are emerging as the standard because they are “just enough” indentation to make the HTML clearly indented to most people's eyes, but because HTML tends to nest very deeply — much more deeply than one would commonly nest when programming — and anything more than two spaces tends to start pushing HTML off the right edge of an 80-column screen pretty quickly.