HTML5 video
Support for HTML5 video has been steadily increasing. In June 2013, Netflix added support for HTML5 video. In January 2015, YouTube switched to using HTML5 video instead of Flash by default. In December 2015, Facebook switched from Flash to HTML5 for all video content.
HTML5
HTML 5 is designed so that old browsers can safely ignore new HTML 5 constructs. In contrast to HTML 4.01, the HTML 5 specification gives detailed rules for lexing and parsing, with the intent that compliant browsers will produce the same results when parsing incorrect syntax. Although HTML 5 now defines a consistent behavior for "tag soup" documents, those documents are not regarded as conforming to the HTML 5 standard.