When it comes to optimizing your website for search engines, every detail matters — including the HTTP headers.
But what exactly are HTTP headers, and why should you care?
HTTP headers allow the browser and the server to exchange important data about a request and response.
This data influences how website content is delivered and displayed to users and impacts everything from security to performance.
Search engines like Google rely on HTTP headers to assess a website’s structure, responsiveness and relevance.
In short, mastering HTTP headers can boost your overall SEO performance. In this article, I’ll cover the basics of HTTP headers and SEO.
HTTP headers are part of a communication framework between a web browser and a server.
They pass along details that help your browser understand how to process and display a website.
Every time you visit a website, a request is sent from your browser to the server hosting that site.
The server responds, sending back the content and HTTP headers that give more instructions.
These headers can include information like the type of content being delivered, whether it should be cached or what security protocols are in place.
The structure of an HTTP header is built on key-value pairs.
Each key tells the browser what kind of information to expect, and the value provides the details.
For example, the header Content-Type: text/html tells the browser that the server is sending HTML code to be displayed as a web page.
When optimizing your website for SEO, there are some HTTP headers to know.
While not an exhaustive list, the following headers help search engines, crawlers and browsers interpret your website correctly.
They can also influence factors like crawling efficiency, content delivery and user experience.
Let’s look at two main categories of HTTP headers: response headers and request headers, and the types of headers to note in each category.
Response headers are sent from the server to the client (which is typically a browser or search engine crawler) and give key information about the resource being delivered.
Status codes
Status codes inform the client of the outcome of the request. Some common codes and their SEO implications include:
200 (OK): Indicates that the request has been successful. This is the ideal response for a functioning page to ensure that it can be crawled and indexed.
301 (moved permanently): Used for permanent redirects. Implementing 301 redirects properly helps preserve SEO value when moving content or consolidating pages as it passes link equity from the old URL to the new one.
404 (not found): Signals that the requested resource doesn’t exist. While common, 404 errors can negatively impact your site’s SEO and user experience. It’s better to redirect users or provide useful 404 pages.
503 (service unavailable): Indicates that the server is temporarily unavailable. When used correctly, such as during maintenance, it tells crawlers that the downtime is temporary, which can prevent issues with indexing.
You can learn more about status codes in my article here on Search Engine Land: The ultimate guide to HTTP status codes for SEO.
Canonical link
The canonical link header helps search engines identify the primary version of a page and is useful for non-HTML files like PDFs or Microsoft Word documents.
Google supports this method for web search results, and it functions similarly to the HTML canonical tag.
Rather than embedding a <link rel=”canonical”> tag in the HTML, you can set the canonical URL in the response header to signal which version of the content should be indexed.
For instance, if you have both a PDF and a .docx version of a white paper, you can use the Link header to specify that the PDF should be treated as the canonical version, as Google illustrates in its documentation:
X-Robots-Tag
This is a flexible header that allows webmasters to control how search engines crawl and index non-HTML resources like PDFs, images and other files.
You can use X-Robots-Tag: noindex to ensure that search engines do not index specific files.
If executed well, it ensures that only the right pages are indexed and shown in search results, preventing things like duplicate content or unnecessary pages appearing in search results.
You can check out Google’s documentation on this header. It gives multiple examples of how to execute the header, like this example:
Here’s an example of an HTTP response with an X-Robots-Tag instructing crawlers not to index a page:
HTTP/1.1 200 OK
Date: Tue, 25 May 2010 21:42:43 GMT
(…)
X-Robots-Tag: noindex
(…)
Strict-Transport-Security (HSTS)
Security-related headers like Strict-Transport-Security (HSTS) are important in securing HTTPS connections.
HSTS ensures that browsers only connect to your site via HTTPS, which enhances both security and user trust.
These headers don’t directly influence search rankings but can have an indirect impact.
As John Mueller pointed out in a June 2023 SEO office-hours video, Google doesn’t use security headers like HSTS as a ranking signal – their primary function is to safeguard users.
That said, having an HTTPS site is still a minor ranking factor, and implementing security headers like HSTS, Content-Security-Policy (limiting the resources a browser can load, which can protect a site from code injection attacks) and X-Content-Type-Options (preventing browsers from guessing file types incorrectly) create a more secure browsing environment.
This protects users and contributes to a more reliable, user-friendly website – a key aspect of long-term SEO success.
Cache-Control
This header manages how resources are cached by browsers and intermediate caches (e.g., CDNs).
A well-implemented Cache-Control header ensures that resources are cached for optimal time periods, which reduces server load and improves page load times, both of which are important for SEO and user experience.
Headers like Cache-Control and Expires ensure that resources that are accessed often are stored locally in the user’s browser and don’t have to be reloaded from the server every time.
Faster load times improve user experience and reduce bounce rates, both of which are signals that Google takes into account when ranking sites.
Content-Type
This header signals the type of content being sent (e.g., HTML, JSON, image files).
The correct Content-Type ensures that browsers and crawlers interpret the content correctly for SEO purposes.
For instance, serving a web page as text/HTML ensures that search engines treat it as HTML content to be indexed.
ETag and Last-Modified
These headers help with content revalidation, which allows browsers to check whether a resource has changed since its last retrieval.
ETag and Last-Modified headers improve load times and reduce unnecessary data transfers and that can positively affect user experience and SEO.
In 2023, Google’s John Mueller explained on Mastodon that getting this tag wrong won’t harm your SEO as some people had thought:
Vary: User-Agent
The Vary: User-Agent header helps deliver the right content by indicating that the version of the resource may change depending on the user’s browser or device.
This helps ensure that the correct version – whether mobile or desktop – is provided to users and cached efficiently.
Mueller clarified on LinkedIn, however, that Google doesn’t rely on Vary: User-Agent headers to distinguish between mobile and desktop versions for SEO purposes.
While the vary header is still useful for enhancing performance and usability by serving the right content and aiding HTTP caches, it doesn’t directly impact how Google processes or ranks your site.
Content-Encoding
The Content-Encoding header indicates if the content being sent from the server to the client (usually a browser) has been compressed.