I did some experiments on your p5.js Web Editor mini-site and these are my discoveries about it: 
- As you already told us the 1st entry page is working as it should.
- However if we open the browser’s console and go to its “Network” tab we won’t spot the file named “styles.css” being loaded there:
editor.p5js.org/bardyl/sketches/J4ZS_c853/styles.css - Instead if we go to “Elements” tab we’ll find the CSS file’s content inside the HTML file at tag
<style>inside tag<head>. - Now clicking at any of those 6 links we’ll get pages w/o any CSS style applied.
- Starting by going to “Console” tab we’ll see the following warning logged:
Resource interpreted as Stylesheet but transferred with MIME type text/html: - Going back to “Elements” tab we can spot the following
<link>tag inside<head>:
<link rel="stylesheet" href="styles.css" type="text/css"> - That didn’t exist on the the 1st entry page which uses a
<style>tag instead. - Investigating it further I went to “Network” tab and clicked at the loaded “styles.css” entry.
- On the 1st tab “Headers” at “Response Headers” I see:
content-type: text/html; charset=utf-8 - But at “Request Headers” I’ve got:
accept: text/css,/;q=0.1 - That means the
<link>tag requests a file of type “text/css” but the server editor.p5js.org returns a “text/html” type instead! - So it’s not your HTML files which are buggy but editor.p5js.org itself doesn’t know how to serve CSS file requests using the proper “text/css” type!
- Instead that online editor resorts on transferring the content of CSS files to its HTML file inside
<style>tags as a workaround! - But that hack fails for the other HTML files linked by the main HTML!
- You may try to bring this glitch to their attention by going here: