HTML stands for Hypertext mark-up language. Helps in building the structure of a page. One can define the layout of the website/landing page.
Meta tags help in SEO for listing our website in a web browser. Widely used all over the world. Versatile and easy to use/learn.
CSS stands for Cascading Style Sheets helps in describing the looks and formatting of our emails, and webpages written in HTML across devices. e.g. Font color, Font size etc. Apart from styling, it is required for responsive designs and reusability.
Responsive designs refer to our emails, and webpages being shown/respond to different devices like PCs, Mobiles, and Tablets according to the change in sizes.
HTML's purpose is to provide the skeleton and CSS's to provide design of the document.