What programming languages are essential for web developers?

HTML, CSS si JavaScript. You can do everything you need from the frontend with only these languages. Then for backend, it depends on what type of application you want. Java, PHP, Python, Ruby on Rails, NodeJs, everything works depending on your requests. 

The must programming languages are essential for web developers are 

HTML And CSS For formatting Language  Markup Language 

JavaScript DOM,jQuery ,AJAX ,JOSN ,AppML For client Side and Validations Language 

JSP,PHP And ASP.net For Server Side Language

MVC,Struts1,Struts2 ,Spring Frameworks  

Canvas ,SVG ,Icons and Google MapsFor Graphics

XML For exchange data 

That's  must languages and technology using web developing

Html/html5,xml for markup,css/css for colouring things,javascript/jquery for transitions and events,php/mysql for server side and dynamic development,ajax/json for static reloading,while frameworks for responsiveness and write less code :)

Many actually... opensource; LAMP, ROR, MEAN,  Python and family etc... closesource: dotNet etc... but to get a headstart, I'd suggest focusing on client side technology would be a good choice E.g. Javascript, HTML, CSS... then progress to using those above mentioned technology.

HTML and CSS are the most important and base language which are must to learn before starting and a web developer should have strong grasp over them.

As the skill set increases they can opt the related knowledge and expertise in advance languages.

Of course , JavaScript ....