Many of ASP.NET server controls base their internal HTML rendering algorithms on the values of their properties. Developers are mostly aware of those properties (not always about their roles in the internal rendering process though) but don't always realize the importance of the default values.
From my experience there is a common assumption that if a property appears empty in the Visual Studio property window that means the default value is empty. Not always true and can be harmful for the output HTML content! Luckily there is always a way to ensure the default values of the properties in ASP.NET markup (.aspx, .ascx). Here are a few examples.
SiteMapPath server control
It has a string property SkipLinkText that by default appears empty in the property window. But in reality if it's not set it's populated from the embedded resource string. That effectively leads to a difference in HTML rendering that does render an <img> tag inside the breadcrumb path with the height and width attributes set to 0 (don't ask me why!).
Some browsers react inadequately on such the HTML if some CSS styling applied to the breadcrumb content. For instance IE6 does not honor a line-height attribute if there is an in line image inside the paragraph.
So to ensure that unnecessary HTML is not rendered in this case always use SkipLinkText="".
ValidationSummary server control
It has a string property ForeColor that defines the color of the text appearing in the control and is prepopulated with the Redvalue. Having a value in this property effectively leads to the control rendering out an inline styling "color:Red;". But if you want to define a text color in the CSS class that you apply to the control you have to explicitly set this property to empty value ForeColor="" otherwise it will always render out the inline styling which will effectively override your CSS settings.
CreateUserWizard server control
This is quite a complex and sophisticated template-based control that comes with ASP.NET Membership framework. From the first sight you assume some degree of intelligence built into the control based on its complexity. For instance, that it does validate user input and render error messages if something is wrong. And it's true except that it does not validate e-mail address string against a regular expression unless a property EmailRegularExpression is set to a proper value and it is not by default. Also it does not provide a developer with such an expression even though it is built into a standard RegularExpressionValidator control.
So be careful and don't forget to set this property EmailRegularExpression="\w+([-+.']\w+)*@\w+([-.]\w+)*\.\w+([-.]\w+)*" otherwise you will be surprised that a user can enter some garbage into the e-mail field and is not stopped doing that.
So above there are just some examples illustrating an importance of paying attention to such details as default values of the server controls' properties. I am sure you can find more examples based on your own experience.
Using .skin File
Also there is another more efficient way to ensure that the critical default values are properly set across a web application: just add all the default values that you want to ensure to your default .skin file. For example, for above mentioned controls add the following lines there:
<asp:SiteMapPath runat="server" SkipLinkText="" />
<asp:ValidationSummary runat="server" ForeColor="" />
<asp:CreateUserWizard runat="server" EmailRegularExpression="\w+([-+.']\w+)*@\w+([-.]\w+)*\.\w+([-.]\w+)*" />
