> Surely the website creator can put this right
Yeah, but it's non-trivial for important reasons.
Here's what is supposed to happen: your computer trusts a bunch of root certificate authorities to sign off on websites. This list is determined by your browser/os/antivirus. Every connection needs to be signed off by one of the authorities. This prevents "man in the middle" attacks where some goober at your ISP or landlord or employer or Starbucks pretends to be amazon.com and tries to steal your credentials / bomb you with ads. How does your browser know if it's talking to the real amazon or goober amazon? By seeing if the certs are signed. The cert authorities don't sign off on goobers.
Here's a common problem: what if a CA gets caught signing off on goobers? Then Google/Microsoft/Malewarebytes pull that CA and all the sites they signed stop working for you until they find another CA. It could be that your browser/os/antivirus revoked the CA but siglent's browser/os/antivirus didn't because China is less concerned about the particular goober than Google/Microsoft/Malewarebytes. In that case the website looks good to siglent but you get a warning as if someone was trying to pull off an attack even though the actual problem is administrative/political. Here's another common problem: if siglent gets a cert that expires in 2 years and then the website administrator for the English domain leaves in 1 year and either forgets to leave a note to renew or the new guy doesn't internalize the need to act on the note, the existing cert expires and boom you get a warning. At least in this case everyone will see the same warning and it should be fixed quickly.
99% of the time if you see one of these warnings it's a false positive and an indication of an administrative/political snafu rather than an actual MITM attack. The language on the cert warnings is dire because if most people condition themselves to bypass the errors, the system stops being an effective deterrent. However, because the language *is* dire and the system *is* an effective deterrent, 99% of the time the right move is to bypass. If you're just window shopping new oscilloscopes, bypassing is probably fine. Just try not to get so complacent that you would bypass and then fill in an important login or payment form.
EDIT: changed the language to not point so squarely at siglent, there's a good chance the snafu didn't start with them.