[Fix] Replace obsolete frameborder attribute on iframes with CSS border property

[qmoreaux]

I want to prequalify a pull request related to the following:

User story:
As a web developer integrating H5P, I want the generated iframe markup to follow current HTML standards so that automated accessibility tools do not flag unnecessary warnings.

Currently, H5P iframes include the obsolete frameborder="0" attribute, which is not part of the HTML5 specification and triggers warnings in tools like axe, WAVE, and Lighthouse.

UX:
No visible changes. The iframe will render identically.

Accessibility:
While not a direct WCAG violation, removing deprecated attributes contributes to cleaner, standards-compliant markup. This supports:

• RGAA 10.1: no deprecated presentational attributes.
• General best practice for HTML5 compliance.

Technical solution:

1. In h5p.js and h5p-resizer.js (or wherever iframes are generated), remove the frameborder="0" attribute from iframe creation.

2. Add border: none; to the inline style or to the CSS stylesheet applied to H5P iframes (h5p.css).

3. Search for all occurrences of "frameborder" across the codebase to ensure complete cleanup.

This is a minimal, low-risk change with no functional impact.

Files likely affected:

• js/h5p.js
• js/h5p-resizer.js
• styles/h5p.css