A extremely simple solution that even a lay person can implement is to use a Google Form . A Google form can receive email and other information, save it to a spreadsheet, and it can then be imported by any newsletter submission service.
Advantages of Google Form (not involving programming)
- Simple to implement
- Fast to implement
- Flexible in creating new fields
- Easy Export
- Does not require prior knowledge of programming
- Free
Disadvantages of Google Form (not involving programming)
- You can not seamlessly integrate your page HTML without the person knowing that you are using Google Forms
Using Google Forms with API help
Limitations of Google Forms without involving programming can be solved by accessing the API directly. See reference for Google Spreadsheets API version 3.0
Addition after question editing
Now that you've radically changed your question, and making the above answer useless, the best you can do is read the Mailchimp's Direct API at link and try to do via REST with pure javascript.
If that alone is not enough, because I simply do not know if a solution is really possible the way you want, you'll end up being simpler to make at least one call on your own server, and that call is accessing APIs of the newsletter providers. That is, your landing page might even be in plain HTML and basic JavaScript, but somewhere, on your own server, or on a server that would intermingle multiple websites, you would have to develop something in some backend language.
It's worth remembering that bulk email providers might not make it much easier to create an easy way to send via HTML because this would allow for a lot of SPAM . And by providing a simple API to do in direct HTML, people would complain about more advanced features, and MailChimp was not meant for that sort of thing.
