The U.S. Securities and Exchange Commission (SEC) has specific guidelines and restrictions for using HTML in EDGAR (Electronic Data Gathering, Analysis, and Retrieval) filings. These guidelines aim to ensure consistency, accessibility, and accuracy in the submitted documents. Please note that regulations may have been updated after my last knowledge update, so it's essential to consult the SEC's official website for the most current information. However, here are some common restrictions and guidelines that were applicable at the time:

File Format: The SEC requires EDGAR filings to be submitted in HTML format, specifically using HTML 4.01 Transitional. Other formats, such as PDF, may not be accepted for certain types of filings.

Inline Styling: The use of inline styling (styling applied directly to HTML elements) should be minimal. Excessive use of inline styling or unsupported CSS (Cascading Style Sheets) properties may not be permitted.

Character Encoding: The character encoding of the HTML document must be specified correctly, typically using UTF-8, to ensure proper interpretation and display of characters.

Metadata: Relevant metadata should be included in the HTML document, such as document title, author, and other relevant information.

Accessibility: The SEC places high importance on accessibility. Filers are required to adhere to accessibility standards, such as the Web Content Accessibility Guidelines (WCAG), to ensure that individuals with disabilities can access the content.

Tables and Tabular Data: When using tables to present financial data or other tabular information, filers must follow specific guidelines to ensure data is correctly presented and accessible.

Javascript and Multimedia: The use of JavaScript, multimedia elements (such as audio or video), and interactive content may be restricted or limited due to potential security concerns or compatibility issues with various systems.

File Size Limitations: The SEC imposes file size limitations for EDGAR filings, so it's essential to optimize the HTML document to keep it within the allowed size.

Validation: Filers are encouraged to validate their HTML documents using online validation tools to ensure compliance with the SEC's formatting requirements.

Hyperlinks and Anchors: Hyperlinks should be used judiciously and should not lead to external websites or resources that may not comply with SEC regulations.

It's crucial for filers to carefully review the SEC's EDGAR Filer Manual and EDGAR Filing Website to understand the specific requirements and restrictions that apply to their filings. Compliance with SEC regulations is critical to ensure the successful submission and acceptance of documents through the EDGAR system.