It all depends on your programming experience but here's probably the best and easiest tech stack you can use:
You will need web hosting, I suggest a simple but reliable managed hosting service that provides cPanel with software installers that include Wordpress, Drupal and others. You can get a site up in minutes.
PHP (back end server) - not actually required but you will need someway to serve your files and perhaps only allow logged in users to access files etc. You can get a simple html canvas up and running to do the dev work but eventually you will need a host and file server anyway.
If you have no experience with PHP, use Wordpress with a webgl plugin or similar (basically a page to hold your 'canvas' to hold your image). ->
https://wordpress.org/plugins/tags/webgl/With a bit of work it wouldn't be too hard to create your own page anyway but that's an option to get going.
If you are comfortable with PHP and want total control then I highly recommend Laravel as your PHP framework, it's very mature with excellent doc's and support etc.
Javascript(JS) (client side) - you will be neck deep in JS from day one, I recommend you stay away from frameworks, most of your code will be plain JS anyway as it pertains to the drawing code not HTML DOM elements. I quite like the 'Module Pattern' to encapsulate my code. You can also use TypeScript but webgl examples are lacking so it may be harder to get going with, it's probably easier to learn than JS but it helps if you know JS to use it.
WebGL - There are many frameworks out there, some high level and some very low. I like three.js and there is probably more sample code using it than any other. Babylon looks good too, both of these are pretty low level. Take your time and check out a few, some have primitive objects ready to draw, some you need to build the primitives from scratch.
TemplatesThis could be done client side or server side, maybe if most of your code is JS then do it client side.
Basically I'd create a sample STEP (or whatever) file of the part you want in your usual CAD app, read carefully through it and note the variables that need to be changed/updated. You might be able to find a template library or build your own but basically you would edit the fields that need changing and give then a variable name. A real basic one would use search and replace, a real simple example below:
Your variables (in some file or hashtable/map of values):
width=200.0
height=100.0
length=1200.0
Your template:
blah bleh {{width}}, {{height}}, {{length}} foo bar baz and whatever other text in this template file.
You then do a search for "{{width}}" and replace it with the width value, rinse and repeat for each value.
You didn't mention your programming ability so this is a simple as it will get, if you have experience with web dev and other languages such as Python or Go then go for it but it takes a lot more work to set things up and look after them.
This isn't a small job, good luck!