HTML stands for Hyper Text Markup Language. It is used to design web pages using markup language. HTML is the combination of Hypertext and Markup language. Hypertext defines the link between the web pages. Markup language is used to define the text document within tag which defines the structure of web pages. HTML 5 is the fifth and current version of HTML. It has improved the markup available for documents and has introduced application programming interfaces(API) and Document Object Model(DOM). To define a client-side script in HTML5, we can do tag or adding external script file by src attribute to the HTML file. <script> tag contains the JavaScript to be added for the client-side scripting. 

Introduction to Client Side and HTML: https://www.geeksforgeeks.org/how-to-define-a-client-side-script-in-html5/  

HTML : https://www.geeksforgeeks.org/html5-introduction/ 

Server side and Client side Programming: https://www.geeksforgeeks.org/server-side-client-side-programming/