Create a new preset in Properties - Render - Armory Exporter and select HTML5 target. Hit Publish to export HTML5 files.
Resulting files will be located at blend_file_location/build_projectname/html5.
Note that you need to copy these files to a web server or start a local server. Otherwise you will get a cross-origin error when opening index.html directly due to security reasons of web browsers. When playing HTML5 project from Blender, Armory starts a local server automatically.
Compared to desktop, a limited set of WebGL extensions is exposed on mobile browsers. To account for this, select the Mobile preset in Properties - Render - Armory Render Path - Preset. Mobile preset is configured to run on every WebGL1 capable device.
Embedding Armory projects into existing web pages is straightforward. Once the project is published, inspect exported index.html file. Take note of the <canvas> and <script> elements.
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8"/>
<title>Armory</title>
</head>
<body style="margin: 0; padding: 0;">
<p align="center"><canvas align="center" style="outline: none;" id='khanvas' width='1280' height='720'></canvas></p>
<script src='kha.js'></script>
</body>
</html>