Yes, you can use HTML on Squarespace, and doing so can offer you greater control over the design and functionality of your website. Here’s a detailed guide on how you can effectively integrate HTML into your Squarespace site, what to consider, and some practical steps to follow:

Where You Can Use HTML in Squarespace:

1. Code Block:
2. Usage: The Code Block is the most straightforward way to add HTML to your Squarespace site. It's designed specifically for embedding custom code.
3. Steps to Add a Code Block:
   1. Navigate to the page where you want to add the code.
   2. In the page editor, click the + icon to add a new block.
   3. Choose the "Code" block from the list of block options.
   4. Enter your HTML (and optionally CSS or JavaScript) in the text area provided.
   5. Save or publish your changes.

4. Considerations: Ensure that your code is clean and tested as poorly written HTML can break page functionality or negatively affect page layout.

5. Code Injection:

6. Usage: Code Injection allows you to add HTML (along with CSS and JavaScript) site-wide or to specific pages, sections like the header or footer.
7. Site-Wide Injection:
   1. Go to Settings -> Advanced.
   2. Select Code Injection.
   3. Add your HTML code in the “Header” or “Footer” section.
   4. Save your changes.
8. Page-Specific Injection:
   1. While editing a specific page, open the page settings.
   2. Go to the Advanced tab.
   3. Insert your HTML in the "Page Header Code Injection" or "Page Footer Code Injection" field.
   4. Save your changes.

9. Considerations: Be cautious with site-wide code injections as they affect every page and can have more widespread implications.

10. Custom HTML for Forms:

11. Usage: If you need more customized forms than what Squarespace's Form Block offers, you can incorporate custom HTML forms.
12. Steps to Add Custom Form HTML:
    1. Create your form HTML code.
    2. Insert a Code Block or use Code Injection to place your form HTML.
    3. Ensure that any server-side form handling (if necessary) is properly set up.

Practical Considerations and Limitations:

1. Security and Performance:
2. Incorrect HTML or scripts can lead to performance issues or vulnerabilities.

3. Avoid using scripts from unreliable sources to prevent security risks.

4. Compatibility:

5. Some advanced HTML features might not render correctly within the constrained environment of a CMS like Squarespace. Always test your custom code extensively across different devices and browsers.

6. Support and Maintenance:

7. Custom code may require regular updates to ensure continued compatibility with Squarespace updates and browser changes.

8. Keep backups of your custom code.

9. Client Restrictions:

10. Be aware that clients might prefer sticking to the more user-friendly aspects of Squarespace. Extensive custom HTML can complicate future edits for non-technical users.

11. SEO Implications:

12. Ensure that your custom HTML is SEO-friendly. Improperly structured code can adversely affect your site's search engine ranking.
13. Use semantic HTML tags and include necessary meta tags.

Conclusion:

Incorporating HTML into your Squarespace site allows for enhanced flexibility and customization. By following the outlined steps and considering the associated risks, you can effectively use HTML to expand the design and functionality of your site. Always remember to test your changes rigorously to maintain a seamless user experience.