There are three main languages you use when you make a website: HTML, CSS, and Javascript.
HTML looks like this:
<!DOCTYPE html>
<html>
<head>
<title>Gravity 1</title>
<script src="./gravity1.js" async></script>
<meta charset="utf-8"/>
</head>
<body>
<canvas id="canvas" tabindex="" width="1000" height="800"></canvas>
</body>
</html>
HTML is the core of a webpage. HTML contains structured text and data and tells the browser where to find the CSS, Javascript, and images for the page. This page is written in HTML.
CSS is used to make webpages look nicer. Ignore it for now.
Javascript is a programming language that browsers can run.
This website is hosted on Neocities. You can go to https://neocities.org/ to create a website for free.
Flash is going to die, but it looks like Scratch 3.0 will not depend on Flash and that "a public alpha release is planned for the first quarter of 2018".