Web guidelines keep everything consistent, from button styles to navigation structure.

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BUTTON HIERARCHY
Create a page that shows what all links do (including the buttons), the appropriate behavior of each and when to use them (with examples of appropriate usage). If one button is dominant, make clear the maximum number of times it should be used per page (usually once at most). Define the hover, disabled and visited states for all buttons.

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ICONS
Defining size and spacing and where to use icons is another great way to promote consistency. the ZURB agency defines icon sizes and when to use them, and it provides clients with an online source from which to download them. ZURB also defines badges and explains their purpose. It believes that its guidelines are best shared online.

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NAVIGATION (LOGGED IN/OUT STATES)
On the Web, good consistent navigation can make or break a website. New pages are often added to a website after the designer is done with it. Have you left some space for this? Doing things like letting people know what to do with new navigation items and showing logged-in states make for a cleaner website.
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BASIC CODING GUIDELINES

There’s no way to make someone else code like you, but you can offer others basic guidelines that will minimize the damage, such as:
CSS class naming conventions
Should they use .camelCase or .words-with-dashes?
JavaScript integration
Are you using jQuery? MooTools? How should new JavaScript be integrated?
Form styling
Include the code, error states and more so that they understand what style conventions you expect.
Doc type and validation requirements
Do you allow certain invalid items? Do you expect the CSS and HTML to validate? Directory structure Make clear how you have organized it.
Accessibility standards
Should people include alt tags? Is image replacement used for non-standard fonts?
Testing methods
Which standard should they test with? Do you have staging and production websites?
Version control
What system are you using? How should they check in new code?