Youtube.com is built with the following tech stack:
The description attribute provides a concise explanation of the page content.
Meta tag containing keywords related to the page.
Open Graph Protocol
The Open Graph protocol enables any web page to become a rich object in a social graph, a open protocol supported by Facebook
Canonical Content Tag
Public specification of a preferred URL for a page allows search engines to understand the original location for content.
Cascading Style Sheets
Cascading Style Sheets (CSS) is a stylesheet language used to describe the presentation of a document written in a markup language. Its most common application is to style web pages written in HTML
The DOCTYPE is a required preamble for HTML5 websites.
HTML 5 Specific Tags
This page contains tags that are specific to an HTML 5 implementation.
X-XSS-Protection is a HTTP header set by Internet Explorer 8+. This header lets domains toggle on and off the "XSS Filter" of IE8, which prevents some categories of XSS attacks.
Content Type Options
Used to disable MIME-sniffing for a particular HTTP response.
Strict Transport Security
The HTTP Strict-Transport-Security (HSTS) header instructs the browser to only use https.
Forces browsers to only communicate with the site using HTTPS.
This page uses the viewport meta tag which means the content may be optimized for mobile content.
Mobile Non Scaleable Content
This content is formatted for mobile devices, it does not allow the content to be scaled.