When do we mix the lenguages?

Question

like, in HTML we use a lot of CSS, but How do I know when I can mix them?

Answer 1

CSS is a formatting language for HTML. Depending on the browser and the element there are certain formating options available to you. In general you can apply some type of CSS to any element. javaScript is used to conduct changes to the elements' position, value, CSS attributes and other attributes based on your code and the users' actions without having to force a page reload.

Answer 2

Suppose you have created 5 Webpages in HTML and if it required to change a property (eg. Heading Color) in all the pages, then changing 1 by 1 on all the 5 pages will be a cubersome task.
Instead if you create a CSS with suitable tags, It is only necessary to change that tag corresponding to heading color and all the 5 pages that you have linked to it will be updated.(moral - Time savings).
Likewise php is used to create dynamic webpages(ie. pages having various forms etc.).