Is there a performance cost for not declaring the "type" attribute in the <style> tag?

From MDN Style docs on the type attribute:

This attribute is optional and defaults to text/css if it's missing.

Since it defaults to the correct one, there is no impact.

You might as well ask if there is a performance cost for making an HTTP request in the first place. It is impossible to completely eliminate the notion of performance itself. At some point you're either at 100% performance efficiency for your page, standards compliant or not. The complimentary aspect to performance is standards in this case. Another example would be entrepreneurialism and execution. All the great ideas in the world are utterly useless unless you can execute to make said business fit into an economic model that works in the real world.

Now it is possible to make great leaps in performance. Back in the 80s and 90s, you could either buy a cheap Honda (non-standards here) or you could improve performance (Porsche, Ferrari or McLaren). Then Tesla came along and completely blew away even the best in acceleration. Disclaimer: I really don't care about cars but it's an easy analogy for most to comprehend.

As someone who has left very few rocks untouched, I can attest to you personally that eventually, it is not you though your environment in which performance increases should be sought. So while I commend you for trying to think outside the box I would make other considerations as to where to improve performance.

• Be wary of too much utilization of the * selector.
• Avoid CSS animation of box-shadow except on ::after and ::before pseudo-elements.
• Be aware of which selector/rules aren't used.
• Be aware to use a single compiled stylesheet for an entire domain you should have very well-honed CSS skills.
• Avoid CSS class puking (class attributes on frigin everything).
• Avoid CSS position absolute and fixed on moderate or minor layout aspects; reserve for only major layout elements.
• Minify your CSS when and wherever possible.
• Reduce the number of HTTP requests.
• Use third-party tools like Yahoo's YSlow and Google PageSpeed Insights for further insight.

It won't have an impact on performance, however older browsers and non standard browsers may misinterpret the content within the elements when the type is not set. I have quite often used embedded blocks with a type element, and haven't seen an issue when using common browsers, IE, Chrome, Firefox or Safari.

As you already said in the your question the default type is 'text/css', which means if the type attribute isn't present the browser uses 'text/css'.

For browsers based on the webkit engine, it's actually faster to omit the type attribute because it first checks for the type to be empty. Therefore if you don't omit the type the browser have to at least do 1 more check. (The check if the provided type is matching text/css).

I could imagine other browsers (not based on webkit) are implementing the css check likely the same. Because for checking an empty string is "easier" from the CPU's point of view (in the case of a webkit based browser it's an AtomicString from the webkit engine).

You can find the css check inside the webkit engine implementation. Can also be found inside the chromium implementation.

According to HTML5 the default type for style is text/css and the default type for the script tag is text/javascript,Even if you dint specify, they will consider the defaults. so that there wont be any performance issue for not mentioning the type considering most modern browsers

The main purpose of the type="text/css" is Set the value of the type attribute to indicate the language of the linked (style sheet) resource. This allows the user agent to avoid downloading a style sheet for an unsupported style sheet language. (www.w3.org)

so basically, it can only reduce the page load by avoiding render blocking files to be downloaded. so its safe to say that, using type attribute to specify the stylesheet type could sometimes increase the pagespeed.

I think it should have an impact, even though it's optional.

Let us consider both cases:

1. Case 1: type is present
   While parsing html document browser will parse style tag and read type attribute and will know the format of the file. It will check the support for type and start download.
2. Case 2: type is not present
   While parsing html document browser will parse style tag and check type attribute.Since it doesn't have a type attribute it will check default type saved in browser settings and try to download the file based on the default settings.

Now the additional step of checking the default settings and applying those should take additional time in case 2. So I would say there's a performance impact but it's really negligible. Specifying type should increase performance.

Since the introduction of HTML 5.x, it's no longer required, if your clients browser have already support HTML5, then you can just omit that one.

This is optional in latest versions, including or excluding the tag does not matter because it is default but including this Tag is not a bad idea because it will help the browsers to identify the script easily- especially old versioned.

Simple answer: Nope :)

It's like a human saying "Hey. How you doing?" instead of "Hey. How are you doing?"... Your brain knows what you trying to say and it doesn't take extra time for you to understand the sentence.

Normally it has no performance cost and with html 5 there should be no problem either its better to not declare it then to set the wrong type

As per MDN

type attribute is optional and defaults to text/css.

So need not to worry about the performance cost, if not declaring type="text/css" in the <style> element?