What are the 10 most important things to learn in web development as of 2018 & why ?

Front-end developer needs to know:

HTML5, CSS3, JavaScript, jQuery
at least one of the front-end JS frameworks/libraries - Angular, Ember, knockout and React,...
at least one of the UI libraries - Semantic UI, Bootstrap, Foundation, Polymer,...
front-end tools - Gulp, Grunt, Bower
One of the CSS preprocessors - LESS, SASS, Stylus
One of the JS preprocessors - CoffeeScript, TypeScript, LiveScript

Back-end developer needs to know:

Basic HTML, CSS, JavaScript
One of the back-end languages - PHP, Python, Ruby,...
One of the back-end frameworks - PHP (Symfony, Laravel, Nette, Zend, CodeIgniter,...), Ruby (Ruby on Rails), Python (django), JavaScript (Node.js, Express.js, Total.js, Sails.js, Meteor.js)
One of the databases - MySQL, MongoDB, PostgreSQL, MariaDB,...

Full-stack developer needs to know:

All of the above (front-end + back-end)
Meteor.js or MEAN stack.

That's correct! HTML, CSS, Javascript all are very important and the basic of web developement. You should know them in detail. These will work in the backend so for front end you should know any of the programming language. Database knowledge is essential for the storing the site related data. MySQL or Oracle or MariaDB etc. any of them should be properly known.

Including these, you should learn basics of the server management so you can at-least setup your account and can resolve the small queries.

Important things to learn in Web Development are:

learn HTML & CSS
Drupal, WordPress and PHP for designing and development of sites

HTML5, Bootstrap apply for multi device
Basic HTML, CSS, JavaScript