Web page layout can be reduced to defining a bunch of containers, applying styles to them and then adding content to said containers. A classic html container is the 'div'.

The following links all generate small sample pages that look identical to each other. But there are many ways to structure the code to accomplish the same effect. For instance, the style block that accompanies most html elements can be placed in one of three areas. It can be 'inline', meaning right with the element it goes with, and in the html body. It can be separated into a style block still that is still within the html document but placed in the head section. Or it can be placed in a completely separate css file. Another important fact is that most everything that can be set up using html elements and styles, can also be done, or undone, or modified by js code. The last example is an extreme case where the html body is completely empty and there is no style info either. The entire page content inside the body is generated by js function calls.

PHP is a computer langauge that can run on the server and allows for more complex behavior in the content served to the user.

A canvas is a rectangular region element in html, like a div, but specifically designed for custom drawing. There is functionality built into js on modern browsers to draw onto a canvas.

Back in the early 1990's most webpages were static - you just looked at them. Now users expect to be able to control content. Interactivity can be obtained by attaching event handlers to various html regions.

The XMLHttpRequest object is built into JS. With it the web page can dynamically send requests back to the server for more content.

A canvas is a rectangular region element in html, like a div, but specifically designed for custom drawing. There is functionality built into js on modern browsers to draw onto a canvas.