Zoompf's Web Performance Blog

Note: Archived Content

This is the archived version of the Zoompf blog. Since our acquisition by Rigor, all our new research and posts on web performance are being published on The Rigor Blog

Performance aspects of Google’s HTML/CSS Style Guide

 Billy Hoffman on April 25, 2012. Category: Optimization

Today Google released their HTML/CSS Style Guide. While it is full of great advice to help manage a growing code base among multiple developers, I thought it would be interested to review the web performance implications of each of its recommendations. Despite being a just style guide, nearly all of its rules had performance ramifications. For each rule I will:

  • Briefly quote Google’s style guide.
  • Link to the rule for more information
  • Discuss how the rule affects performances (either positively or negatively)
  • Mention any other related performance rules, and whether Zoompf’s free or paid offering currently detects the performance issue.

Let’s get started!

Omit the protocol from embedded resources.

Omit the protocol portion (http:, https:) from URLs pointing to images and other media files, style sheets, and scripts unless the respective files are not available over both protocols. (link)

This is a great recommendation for performance. I’ve talked about the advantages of protocol relative URLs in the past. Not only do they both reduce page size, protocol relative URLs prevent you from needlessly creating SSL connections. As an added benefit, protocol relative URLs are a bulletproof way to prevent mixed content warnings.

Applicable Zoompf Checks:

  • SSL Resource on non-SSL Page
  • Excessive Absolute URLs (Same Site)
  • Excessive Absolute URLs (External Links)

Bonus Performance Tip. While this advice is about linking resources, Zoompf recommends to you relative URLs for web content. So instead of linking to https://zoompf.com/blog/some-awesome-post you would use /blog/some-awesome-post. Not only does this reduce page size, but makes it easier to transfer content to new hostnames or restructure your site in the future.

Indent by 2 spaces at a time

Indent by 2 spaces at a time. Don’t use tabs or mix tabs and spaces for indentation. (link)

More of a style rule than performance. You should be minifying your HTML when you move it into production anyway which removes unneeded whitespace and resolves this problem.

Applicable Zoompf Checks:

  • Dynamic HTML Not Minified
  • Static HTML Not Minified

Use only lowercase

All code has to be lowercase: this applies to element names, attributes, attribute values (unless text/CDATA), selectors, properties, and property values (with the exception of strings). (link)

Using lowercase in HTML makes your pages more compressible, resulting in smaller pages.

Zoompf does not check specifically for this issue but does normalize your HTML to lowercase as part of our HTML minification analysis.

Trailing whitespace

Trailing white spaces are unnecessary and can complicate diffs. (link)

A good style rule, and since the document mentions diffs, this is mainly to make internal code management easier. As I said before, post processing your HTML, or minifying it when promoting it to the production environment resolves this issue and can slightly reduce the size of a page.

Applicable Zoompf Checks:

  • Dynamic HTML Not Minified
  • Static HTML Not Minified

Use UTF-8 (no BOM)

Make sure your editor uses UTF-8 as character encoding, without a byte order mark. Specify the encoding in HTML templates and documents via <meta charset="utf-8">. Do not specify the encoding of style sheets as these assume UTF-8. (link)

Standardizing on a single encoding is a good idea, and getting ride of the BOM saves 3 bytes. However I don’t suggest using anything inside the HTML, like <meta charset="utf-8">, to specify the charset. The Content-Type header should be the only thing you use to tell the browser about the charset. Encoding inside the document what the encoding format for a document is creates a chicken or the egg situation where the browser must parse the document to figure out how to parse the document. This creates performance problems and exposes you to XSS attacks. Google’s own documentation even references Zoompf for this.

Applicable Zoompf Checks:

  • Unnecessary <meta> Character Set (http-equiv)
  • Unnecessary <meta> Character Set (charset)


Explain code as needed, where possible. (link)

Comments are absolutely important, but they should not make it into production code. They needlessly increase the size of the resource, and often expose you to security risks.

For things like CSS and JavaScript, minification will remove the comments for your automatically. For HTML, especially dynamically generated HTML, consider using the code comments in the server-side language instead of HTML comments.

So instead of this:

    <a href="/public/">public</a>
    Feature is not ready yet, disable until next release cycle
    <a href="/public/">public</a>

Replace it with something like this:

    <a href="/public/">public</a>
    Feature is not ready yet, disable until next release cycle
    <a href="/public/">public</a>

Applicable Zoompf Checks:

  • Bloated HTML Tag (<comment>)
  • Excessive HTML Comments
  • Commented Out HTML
  • Dynamic HTML Not Minified
  • Static HTML Not Minified
  • JavaScript Not Minified (External File)
  • CSS Not Minified (External File)
  • JavaScript Not Minified (<script>)
  • CSS Not Minified (<style>)
  • Feed Not Minified

Action items

Highlight todos by using the keyword TODO only, not other common formats like @@. Append a contact (username or mailing list) in parentheses as with the format TODO(contact). (link)

TODOs are a quick way to remind you of additional tasks. Just make sure that TODOs are not included in content sent to the user. Since TODOs are placed inside of comments minification fixes this issue.

Zoompf does not check for TODO items specifically, but this issue will be flagged more generically by Zoompf comments and minification checks.


HTML5 (HTML syntax) is preferred for all HTML documents: ‘<!DOCTYPE html>’. (It is recommended to use HTML, as ‘text/html’. Do not use XHTML. XHTML, as ‘application/xhtml+xml’, lacks both browser and infrastructure support and offers less room for optimization than HTML.) (link)

Smaller DOCTYPE means smaller bytes. XHML is also more bloated than HTML, requiring you to use closing tags and <CDATA> which [needlessly increases the size of your page](http://perfectionkills.com/optimizing-html/). Avoid XHTML, especially on mobile sites.

Zoompf does not currently check for HTML5 use, or mobile sites using XHTML.

Use HTML according to its purpose

Use elements for what they have been created for. (link)

Google’s example for this rule shows a <div> using a onclick to replicate a standard <a href=""> tag. Another common example is that developers with use an <a> tag with an empty href attribute, but use an inline onclick handler which simply navigates to a web page.Reinventing the wheel like that almost always results in larger markup, increase the size of the page.

Zoompf does not check for using elements properly.

Separate structure from presentation from behavior

Strictly keep structure (markup), presentation (styling), and behavior (scripting) apart, and try to keep the interaction between the three to an absolute minimum. (link)

Placing JavaScript and CSS into separate external files allows you to reuse common content and reduce the size of all of your pages. These external files can also be cached while HTML often cannot.

This is another good example of advice that is good for development and not good for production. Despite the guide advice, Developers can and should separate code into files as it logically makes sense, such as separating libraries. The same is true for CSS. However, you should not reference 8 JavaScript files in the HTML when the website moves to production. Those files should be combined and versioned as part of the publishing process.

Bonus Tip. Smaller files should be inlined into the document. Software like mod_pagespeed can do this automatically.

Applicable Zoompf Checks:

  • Common Style Attribute
  • Common Content (JavaScript Block)
  • Common Content (Style Block)

Do not use entity references

There is no need to use entity references like &mdash;, &rdquo;, or &#x263a;, assuming the same encoding (UTF-8) is used for files and editors as well as among teams. (link)

All of these symbols appear in UTF-8, so there is no reason to use HTML entities, reducing the size of your page.

Zoompf does not currently check for using entity references on UTF-8 encoded pages.

Omit optional tags (optional)

For file size optimization and scannability purposes, consider omitting optional tags. (link)

I disagree with this advice. While it makes sense to omit end tags of elements that have no meaningful inner HTML (<meta>, <link>, <img>, <script src="">), you are just asking for maintainence issues if you start omitting things like </p>. In fact, Google’s example of recommended HTML usage is actually a better example why this is a stupid practice.

If you truly want to do something like this, don’t try to do it in your source HTML or template files. Add these aggressive HTML minification optimizations as part of your publishing process when moving a website to a production environment.

Zoompf does not check for optional end tags being used.

Use valid CSS where possible

Unless dealing with CSS validator bugs or requiring proprietary syntax, use valid CSS code. (link)

CSS minification tools are very primitive. While JavaScript minifiers like YUI Compressor or Closure Compiler actually tokenize, parse, and build syntax trees to guide minification, CSS minifiers use regular expressions or primitive lexers. Invalid CSS often breaks these tools. Make sure your CSS is valid.

While Zoompf does not check for invalid CSS syntax, it does check to make sure that CSS resources referenced with a <link> tag or @import statement are, in fact, CSS files.

Use appropriate lengths for ID and class names

Use ID and class names that are as short as possible but as long as necessary. Try to convey what an ID or class is about while being as brief as possible. (link)

Shorter names are better than long names for performance. Zoompf does not currently check for long ID or class names.

Avoid qualifying ID and class names with type selectors

Unless necessary (for example with helper classes), do not use element names in conjunction with IDs or classes. (link)

In other words, ul#ref-list is not needed when #ref-list will do. This has three performance implications. First, simplifying how CSS selectors match speeds up rendering. Additionally, the rule is more generic, allowing it to be reused in more situtations and prevents the creation of redundant rules. Finally, smaller selectors mean smaller CSS files, which is always good for performance.

Zoompf does not currently check the specificity of CSS rules.

Omit Default or Unneeded Values

Google’s document lists a number of rules for omiting default or unneeded values from CSS. All of these rules lead to smaller CSS files, which is a good thing for performance. All of these rules can be automatically fixed with CSS minification.

Applicable Zoompf Checks:

  • CSS Not Minifed (External File)
  • CSS Not Minifed (<style>)


Every organization should use a style guide. For the most part, Google’s HTML/CSS Style Guide promotes good style advice that is also good performance advice, making it a good standard to use. Just remember, you should not make things easy for developers and the expense of your visitors. At the same time, don’t force a style on the developers that makes code or content harder to undestand for the sake of performance. Instead, use a build process to apply many of these optimizations automatically.

Want to see what performance problems your website has? I’ve mentioned only a few of the nearly 400 performance issues Zoompf detects when testing your web applications. You can get a free performance scan of you website now and take a look at our Zoompf WPO product at Zoompf.com today!


Have some thoughts, a comment, or some feedback? Talk to us on Twitter @zoompf or use our contact us form.