The HTML Tag
Visual Studio won't do this for you automatically, but in the new framework version there is a new class to specifcally represent the HTML tag.
In the previous version, your HTML tag would have been a System.Web.UI.HtmlControls.HtmlGenericControl
In the new version it needs to be a System.Web.UI.HtmlControls.HtmlElement
The IFRAME Tag
Same scoop as above.
In the previous version, an IFRAME tag would have been a System.Web.UI.HtmlControls.HtmlGenericControl
In the new version it needs to be a System.Web.UI.HtmlControls.HtmlIframe
*.config
Not so much a "breaking change", but something to be aware of as it affects any overrides to web.confg
The new framework version requires changes to config files.
app.config
Previous versions specified the supported runtime using:
<supportedRuntime version="v4.0" sku=".NETFramework,Version=v4.0"/>
In the new version, it is specified using:
<supportedRuntime version="v4.0" sku=".NETFramework,Version=v4.5.2"/>
web.config
Similarly, the compilation/debug configuration element needs to specify the target framework.
Previous versions specified as:
<compilation defaultLanguage="c#" debug="true" targetFramework="4.0"/>
In the new version, it is specified as:
<compilation defaultLanguage="c#" debug="true" targetFramework="4.5.2"/>