You have mentioned EU safety regulations many times. Is there any more technical name for it? I couldn't find it
CE mark stands for
Conformité Européenne - European Conformity - compliance with European safety standards
Essentially it works like this: you build a product based on safety norms for electrical or mechanical devices etc that apply to your type of product/device, and state compliance by putting CE mark on it.
The norms though are a pretty annoying part of this whole safety thing as you need to buy them and they get updated once in awhile or replaced by newer ones and you're supposed to use the most recent ones applicable when designing a new product and stating CE for it. They also often refer to other norms, so you need to have those to make sure what they are referring to in specific situations.
For products you sell to consumers, the general idea is that if the user/anyone in vicinity didn't read instructions or doesn't follow instructions, it shouldn't possess risk of injury or damage to the mechanism to a reasonable degree - for example you can put huge holes for unprotected fans, but a child or a cat will try to play with that fan for example, so big holes are a no-go and I've explained this a few times already, in various places. But something like dropping whole PC on your feet isn't what you can completely prevent as a manufacturer, so what applies here is whether it's not too easy to accidentally tip over where positioned vertically etc.
For businesses you may sell products that are either without CE or state CE with manual that states for example that trained operator is required and it shouldn't be used outside what's stated in the manual and so on. So for example for devices like this, it means that if some business wants to buy something like it, and there's an employee that will work with such device and get injured because he didn't get an actual safety training with whole paper trail etc, he can go to court. Also when it's about damaging the device because the employee wasn't trained correctly etc.
So for example we could make an open bench test style frame and state CE for it because it wouldn't pose a risk on its own, but you probably couldn't state CE compliance for the whole system on such open frame, without stating this to be used by skilled electrician (in the work environment), so if a business wants to buy such systems for example for office workers to have a stylish element in it, technically they shouldn't allow people from outside to the vicinity of those machines and again should have safety training for new employees if not require a secretary to be an electrician if the norms say so about open access to device under operation.
If we want our case to be sold at retail at some point and we do want it, it needs to be made according to this safety. Consider situation where a store has an option to put together whole system and a business buys it from them. If we don't state CE, so not all components required to do so have CE marks, then the store is responsible for stating the CE and they won't sell such system. If we put in instructions stuff like "only to be operated by trained personnel" etc, then at some point the business that bought it will get back to the seller for not informing on the website that this is not a consumer friendly device, or maybe even the retailer will not accept such product after checking the samples.
TLDR - not a simple topic, essentially selling a consumer device in EU requires it to be safe not only to the user that read and follows the instructions, and there are a lot of "rules of a thumb" on how to do things and how to approach to designing new things that are not necessarily specified, and a lot of that stuff is in the EU safety norms.
Rule of a thumb for vent holes or any other openings in device enclosure is that you can't touch the devices components that are doing the work through them, so you can't touch circuitry or spinning fans inside etc. For a big device it may be protected by a fence in a way that you can put whole hand through the fence so a grown man doesn't reach the mechanism this way, but for a small device it's that you shouldn't put the finger through the hole if the PCB or fan is right behind it. And at this point it also applies to the finger size of the child that just started walking, so for example you can have a hole that the kid will fit finger through, but the distance to the fans and PCB is big enough to not reach it, but an adult or teenager won't fit the finger through because he could reach the mechanism.
And there's of course stuff like electrical grounding, isolating wires and connectors, deburring sharp edges etc, but most of that is industry standard, so other PC case start-up can't go wrong there since it would look outright poorly and get backlash from reviewers right away, but when it comes to perforation on small cases, some of them are going for big holes that are not safe and even boast "the super patented hole design" when it's simply not safe, but most people won't notice it until something bad actually happens.
For a small startup making one type of product in limited quantities, having a risk of lawsuit over safety omissions might be okay and part of the business, but for a company like ours, that designs machinery that literally can kill if not designed and operated properly, making a PC case not to the safety standards would mean that if we can't design such simple product according to safety standards, why a potential industrial client should believe that we will do it properly with bigger projects. For big case vendors it's a bit different, but they still won't risk having to recall thousands of cases if something were to be designed not to safety standards and also not getting back the investment in expensive mass production tooling as well.
