# Google HTML CSS Style Detailed > Comprehensive Google HTML/CSS style rules with section-level guidance. Use when google-html-css-style is insufficient, for thorough reviews, or when the user names this skill. - Skill: `peterish8/google-html-css-style-detailed` (Agent Skill, multi-file: 2 files) - Install (CLI): `npx skillmds@latest add peterish8/google-html-css-style-detailed` - Raw SKILL.md: https://api.skillmd.com/api/skills/peterish8/google-html-css-style-detailed/raw - Safety review: pending - Works with: Claude Code, Claude.ai, OpenAI Codex - Category: Web & Frontend - Author: peterish8 (https://skillmd.com/u/peterish8) - Updated: 2026-09-21 - Page: https://skillmd.com/skills/peterish8/google-html-css-style-detailed --- # Google HTML/CSS Style (Detailed) Source: [https://google.github.io/styleguide/htmlcssguide.html](https://google.github.io/styleguide/htmlcssguide.html) Pair with the quick reference: [google-html-css-style](../google-html-css-style/SKILL.md) This skill expands the short skill with section-level rules from Google's guide. For authoritative full text, see the official URL and [reference.md](reference.md). ## How to use 1. Start from [google-html-css-style](../google-html-css-style/SKILL.md) for everyday edits 2. Use this skill for reviews, ambiguous cases, or API/doc conventions 3. Consult [reference.md](reference.md) for source paths and supplemental docs ## Section rules #### Protocol - Use HTTPS for embedded resources where possible. - Always use HTTPS (https:) for images and other media files, style sheets, and scripts, unless the respective files are not available over HTTPS. #### Indentation - Indent by 2 spaces at a time. - Don’t use tabs or mix tabs and spaces for indentation. #### Capitalization - All code has to be lowercase: This applies to HTML element names, attributes, attribute values (unless text/CDATA), CSS selectors, properties, and property values (with the exception of strings). #### Trailing Whitespace - Remove trailing white spaces. - Trailing white spaces are unnecessary and can complicate diffs. #### Encoding - Make sure your editor uses UTF-8 as character encoding, without a byte order mark. - Specify the encoding in HTML templates and documents via . Do not specify the encoding of style sheets as these assume UTF-8. - (More on encodings and when and how to specify them can be found in Handling character encodings in HTML and CSS.) #### Comments - Explain code as needed, where possible. - Use comments to explain code: What does it cover, what purpose does it serve, why is respective solution used or preferred? - (This item is optional as it is not deemed a realistic expectation to always demand fully documented code. Mileage may vary heavily for HTML and CSS code and depends on the project’s complexity.) #### Action Items - Mark todos and action items with TODO. - Highlight todos by using the keyword TODO only, not other common formats like @@. - Append action items after a colon as in TODO: action item. #### Document Type - Use . - Always put your HTML in no-quirks mode by including at the beginning of the document. - A document without a doctype is rendered in “quirks mode”, and one with a different doctype may be rendered in “limited-quirks mode”. These modes don’t follow the widely-understood, widely-documented behavior for various core HTML and CSS constructs, and are likely to cause subtle failures and incompatibilities especially when re-using code that expects no-quirks mode. #### HTML Validity - Use valid HTML where possible. - Use valid HTML code unless that is not possible due to otherwise unattainable performance goals regarding file size. - Use tools such as the W3C HTML validator to test. - Using valid HTML is a measurable baseline quality attribute that contributes to learning about technical requirements and constraints, and that ensures proper HTML usage. #### Semantics - Use HTML according to its purpose. - Use elements (sometimes incorrectly called “tags”) for what they have been created for. For example, use heading elements for headings, p elements for paragraphs, a elements for anchors, etc. - Using HTML according to its purpose is important for accessibility, reuse, and code efficiency reasons. #### Multimedia Fallback - Provide alternative contents for multimedia. - For multimedia, such as images, videos, animated objects via canvas, make sure to offer alternative access. For images that means use of meaningful alternative text (alt) and for video and audio transcripts and captions, if available. - Providing alternative contents is important for accessibility reasons: A blind user has few cues to tell what an image is about without @alt, and other users may have no way of understanding what video or audio contents are about either. - (For images whose alt attributes would introduce redundancy, and for images whose purpose is purely decorative which you cannot immediately use CSS for, use no alternative text, as in alt="".) #### Separation of Concerns - 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. - That is, make sure documents and templates contain only HTML and HTML that is solely serving structural purposes. Move everything presentational into style sheets, and everything behavioral into scripts. - In addition, keep the contact area as small as possible by linking as few style sheets and scripts as possible from documents and templates. - Separating structure from presentation from behavior is important for maintenance reasons. It is always more expensive to change HTML documents and templates than it is to update style sheets and scripts. #### Entity References - Do not use entity references. - There is no need to use entity references like —, ”, or ☺, assuming the same encoding (UTF-8) is used for files and editors as well as among teams. - The only exceptions apply to characters with special meaning in HTML (like < and &) as well as control or “invisible” characters (like no-break spaces). #### Optional Tags - Omit optional tags (optional). - For file size optimization and scannability purposes, consider omitting optional tags. The HTML5 specification defines what tags can be omitted. - (This approach may require a grace period to be established as a wider guideline as it’s significantly different from what web developers are typically taught. For consistency and simplicity reasons it’s best served omitting all optional tags, not just a selection.) #### type Attributes - Omit type attributes for style sheets and scripts. - Do not use type attributes for style sheets (unless not using CSS) and scripts (unless not using JavaScript). - Specifying type attributes in these contexts is not necessary as HTML5 implies text/css and text/javascript as defaults. This can be safely done even for older browsers. #### id Attributes - Avoid unnecessary id attributes. - Prefer class attributes for styling and data attributes for scripting. - Where id attributes are strictly required, always include a hyphen in the value to ensure it does not match the JavaScript identifier syntax, e.g. use user-profile rather than just profile or userProfile. - When an element has an id attribute, browsers will make that available as a named property on the global window prototype, which may cause unexpected behavior. While id attribute values containing a hyphen are still available as property names, these cannot be referenced as global JavaScript variables. #### General Formatting - Use a new line for every block, list, or table element, and indent every such child element. - Independent of the styling of an element (as CSS allows elements to assume a different role per display property), put every block, list, or table element on a new line. - Also, indent them if they are child elements of a block, list, or table element. - (If you run into issues around whitespace between list items it’s acceptable to put all li elements in one line. A linter is encouraged to throw a warning instead of an error.) #### HTML Line-Wrapping - Break long lines (optional). - While there is no column limit recommendation for HTML, you may consider wrapping long lines if it significantly improves readability. - When line-wrapping, each continuation line should be indented to distinguish wrapped attributes from child elements. Lines should be wrapped consistently within a project, ideally enforced by automated code formatting tools. #### HTML Quotation Marks - When quoting attributes values, use double quotation marks. - Use double ("") rather than single quotation marks ('') around attribute values. #### CSS Validity - Use valid CSS where possible. - Unless dealing with CSS validator bugs or requiring proprietary syntax, use valid CSS code. - Use tools such as the W3C CSS validator to test. - Using valid CSS is a measurable baseline quality attribute that allows to spot CSS code that may not have any effect and can be removed, and that ensures proper CSS usage. #### Class Naming - Use meaningful or generic class names. - Instead of presentational or cryptic names, always use class names that reflect the purpose of the element in question, or that are otherwise generic. - Names that are specific and reflect the purpose of the element should be preferred as these are most understandable and the least likely to change. - Generic names are simply a fallback for elements that have no particular or no meaning different from their siblings. They are typically needed as “helpers.” - Using functional or generic names reduces the probability of unnecessary document or template changes. #### Class Name Style - Use class names that are as short as possible but as long as necessary. - Try to convey what a class is about while being as brief as possible. - Using class names this way contributes to acceptable levels of understandability and code efficiency. #### Class Name Delimiters - Separate words in class names by a hyphen. - Do not concatenate words and abbreviations in selectors by any characters (including none at all) other than hyphens, in order to improve understanding and scannability. #### Prefixes - Prefix selectors with an application-specific prefix (optional). - In large projects as well as for code that gets embedded in other projects or on external sites use prefixes (as namespaces) for class names. Use short, unique identifiers followed by a dash. - Using namespaces helps preventing naming conflicts and can make maintenance easier, for example in search and replace operations. #### Type Selectors - Avoid qualifying class names with type selectors. - Unless necessary (for example with helper classes), do not use element names in conjunction with classes. - Avoiding unnecessary ancestor selectors is useful for performance reasons. #### ID Selectors - ID attributes are expected to be unique across an entire page, which is difficult to guarantee when a page contains many components worked on by many different engineers. Class selectors should be preferred in all situations. #### Shorthand Properties - Use shorthand properties where possible. - CSS offers a variety of shorthand properties (like font) that should be used whenever possible, even in cases where only one value is explicitly set. - Using shorthand properties is useful for code efficiency and understandability. #### 0 and Units - Omit unit specification after “0” values, unless required. - Do not use units after 0 values unless they are required. #### Leading 0s - Always include leading “0”s in values. - Put 0s in front of values or lengths between -1 and 1. #### Hexadecimal Notation - Use 3 character hexadecimal notation where possible. - For color values that permit it, 3 character hexadecimal notation is shorter and more succinct. #### Important Declarations - Avoid using !important declarations. - These declarations break the natural cascade of CSS and make it difficult to reason about and compose styles. Use selector specificity to override properties instead. #### Hacks - Avoid user agent detection as well as CSS “hacks”—try a different approach first. #### Declaration Order - Alphabetize declarations (optional). - Sort declarations consistently within a project. In the absence of tooling to automate and enforce a consistent sort order, consider putting declarations in alphabetical order in order to achieve consistent code in a way that is easy to learn, remember, and manually maintain. - Ignore vendor-specific prefixes for sorting purposes. However, multiple vendor-specific prefixes for a certain CSS property should be kept sorted (e.g. -moz prefix comes before -webkit). #### Block Content Indentation - Indent all block content. - Indent all block content, that is rules within rules as well as declarations, so to reflect hierarchy and improve understanding. #### Declaration Stops - Use a semicolon after every declaration. - End every declaration with a semicolon for consistency and extensibility reasons.