He's saying that if you depend on A and A depends on B and they're both MIT licensed then technically you should include the licenses for A and B on your site, but most people only include A.

Surely the MIT license only mentions leaving the notice intact, It doesn't say anything about including the notice on your website.

"The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software."

If you are using some js library with the MIT license in your frontend then you are distributing a copy of the software. You have to include the notice somewhere on your website (typically next to the library)

People viewing your website don't get a copy of the software, nor do they get a substantial portion. The MIT license (like most licenses) applies to the code, not to what that code produces.

If people are downloading the source, and executing that code, eg as parent said "frontend", then yes, it's on their computer. It's not server side, such as with PH, or python or backend js.

However, the license doesn't have to appear on the website portion specifically highlighted for human viewing. It merely needs to exist, say, at the top of every javascript file.

Or a link to where the text form of the license is.

Just because it's inconvenient, and just because people are ignoring it, doesn't mean it's being done as it should be...

My point is that if you install the code through a dependency of a dependency it should be leaving license notices entirely intact, fulfilling the license.

Of course javascript optimizers and the like might throw a wrench in the works these days, but that's a pretty recent and specific problem.

Is that minification?

I thought minifiers were supposed to keep licenses (they can detect the license header). I suppose this could get turned off or messed up by accident with the gigantic custom webpack bundlers even stuff like create-react-app ship with.

Lots of libraries don't put the license in the header, only in a separate LICENSE file.

I've looked at Webpack's output before, and I don't recall seeing any headers there.

Webpack definitely keeps the license headers even when minification is on. They have to start with something like "@license".