咨询：能否开发可在side panel加载HTML页面的Gmail add-on？

Great question! The short answer is yes, you can build a Gmail Add-on that loads external HTML content in the side panel, but there are some key constraints and best practices to keep in mind—especially compared to Outlook’s more straightforward iframe support.

Key Constraints & Workable Solutions #

Gmail Add-ons prioritize security, so direct full-site embedding has limitations, but these workarounds will get you close to your goal:

1. Embed via Iframe (Recommended If Possible) #

If your target site (someresource.com) allows iframe embedding (i.e., it doesn’t set restrictive X-Frame-Options headers or a Content Security Policy that blocks framing), this is the simplest approach. Use Google Apps Script’s HtmlService to wrap the site in an iframe and load it in the sidebar.

Here’s a quick code example:

function showResourceSidebar() {
  const sidebarHtml = HtmlService.createHtmlOutput(`
    <iframe src="https://someresource.com" 
            style="width:100%; height:600px; border:none;" 
            frameborder="0">
    </iframe>
  `).setTitle('My Resource Site');
  GmailApp.getUi().showSidebar(sidebarHtml);
}

A few things to note:

• The site must use HTTPS (Gmail blocks HTTP content in add-ons for security).
• If the site requires login, the iframe will maintain its own session—users may need to log in separately within the sidebar unless you implement OAuth integration (a more complex step).

2. Fetch & Render Static Content (If Iframes Aren’t Allowed) #

If someresource.com blocks iframe embedding, you can use UrlFetchApp to pull the site’s static HTML, sanitize it, and display it in the sidebar. However, this has major tradeoffs:

• Dynamic elements (like interactive forms, JavaScript-driven features, or real-time updates) will likely break, since you’re only loading static markup.
• Site styles may be broken if external CSS isn’t loaded correctly.
• You’ll need to sanitize the fetched HTML to avoid security risks (use HtmlService.createHtmlOutputFromBlob or trusted sanitization libraries).

3. Build a Native Card Experience (If Full Site Isn’t Critical) #

If embedding the entire site isn’t necessary, consider using Gmail’s Card Service to build a native sidebar that pulls key data from someresource.com via its API (if available). This creates a more polished, integrated experience than an iframe, but requires the site to have an accessible API.

Comparing to Outlook #

You’re right that Outlook plugins offer more flexible iframe support with fewer restrictions. Gmail’s approach is more security-focused, which is why you’ll need to work within the above constraints—but the core functionality you want is still achievable.

Important Tips #

• Compliance: Make sure your add-on follows Google’s Workspace Add-on Policies, especially around content security and user privacy.
• Permissions: If your add-on fetches external content, declare the appropriate scopes in your appsscript.json manifest (e.g., https://www.googleapis.com/auth/script.external_request for UrlFetchApp).
• Testing: Always test with your target site—some sites block framing by default, so if you control someresource.com, you may need to adjust its CSP settings to allow embedding.

内容的提问来源于stack exchange，提问作者vzhemevko