I consider to build a website without any JavaScript/CSS.
I want it to be the most light weight a website can be ever.
Assuming that the markup would be fine, is it an SEO benefit or actually an SEO troublemaker (Google).
I'm assuming that your website must be focused entirely around static text content, like a blog. That type of website doesn't need any special treatment to be readable by web crawlers. I would expect a lightweight website like that to score great for on-page SEO metrics like page load time and (obviously) page weight. In other words, there would be nothing preventing your website from ranking well - your site's SEO would stand entirely on its content and its backlink profile.
Regarding CSS, nowadays I wouldn't recommend trying to go with zero CSS because the long line lengths can make the page unreadable, but you could always inline a small CSS library to fix the line length and create some visual space. The most succinct library I've seen is the following - one selector with three rules:
body {
max-width: 38rem;
padding: 2rem;
margin: auto;
}
I tend to read a lot of software development blogs that are very plain in styling, and none of them seem to have trouble ranking. If you want some inspiration, check out Richard Stallman's personal site (founder of GNU and Free Software Foundation).
A simple HTML site like you are talking about in theory should do great as far as SEO goes because of how light-weight it will be. I think one problem you will run into is not using any CSS because its going to be tricky to have your site be desktop, tablet, and mobile friendly without using at least a little CSS.
The other issue you might have is optimization. Since you wont be using images or any other type of content you will need to make sure your HTML is written really well and has really great alt tags, naming convention, and formatting. You could use schema to help with this but then technically you would be using JS.
A website like the one you describe would have some SEO benefits, since the speed is a SEO ranking factor. This example means a single HTML document with small size (some kbytes), so you would also have only one http-request, increasing the TTFB (time to first byte). This website would have an advantage over other websites; most of the CMS (WordPress, Drupal...) have a higher TTFB because they have to process the PHP before to render it in the web browser.
However, a website like this would have a bad user experience, because it wouldn't be responsive, with an ugly design, without interactivity, what also it's bad in terms of accessibility. Since the user experience is a SEO ranking factor, this would be bad for that case.
A good idea is to have a good balance between design and the resources that you include in the website. For example, here is where the static content generators win (Jekyll, Hugo, Gatsby, VuePress, Pelikan, and so on). In this cases, you generate the website before the deployment (upload it to the server). They are in essence static HTML files, so they have a fast TTFB, they don't need to process PHP (less CPU usage), so they can also handle more concurrent visitors (at the same time).
The best for SEO is the highest speed you can get while keeping a good design for the user experience. Think always with the user.
I designed a simple html website with some help from AI. Its response time in Google crawl report is only 57ms compared to hundreds or more ms in case of other websites. It started driving impressions within a week. 57ms is not just superfast response time, it is like a big plus sign for your website SEO. If you want to check out, let me know. If you can handle it, making an html website is great from SEO point of view. It needs just a little extra work to add schema manually. I have also made a website in hugo but that too does not beat the the html website.
