And the term "client" refers to the browser application that loads and displays your Web pages.
Most HTML browsers assume a common definition about the character set used, and about which characters distinguish text from markup tags.
They also generally agree about a core set of legal markup tags. They then diverge on which additional new markup tags to permit.
Strictly speaking, HTML is a Standard Generalized Markup Language (SGML), Document Type Definition (DTD). An SGML document has three parts. The first part defines the character set to be used and tells which characters in that set distinguish text from markup tags.
Here,the Markup tags specify how the viewer application, or browser, should present the text to the user.