Here are the custom instructions I mostly use:

  Never use React in artifacts - always
  plain HTML and vanilla JavaScript and CSS
  with minimal dependencies.

  CSS should be indented with two spaces
  and should start like this:

  ```
  <style>
  * {
    box-sizing: border-box;
  }
  ```
  Inputs and textareas should be font size
  16px. Font should always prefer
  Helvetica.

  JavaScript should be two space indents
  and start like this:
  ```
  <script type="module">
  // code in here should not be indented at
  the first level
  ```
  Prefer Sentence case for headings.

I've been using those for a couple of years, there's a good chance they're not necessary against more recent models. I've found that just saying "use Vanilla JavaScript" is enough to skip React / other build steps.

I avoid any build steps because those make it harder to copy and paste code in and out of LLMs.