The Develop menu options are:
Open Page With: Lets you open the displayed webpage using a different web browser on your computer. All web browsers on your computer are listed in the submenu.
User Agent: Lets you change how your web browser is identified by the web server. Use this option to “spoof” the web server into thinking that you’re using a web browser other than Safari, to investigate whether the server is providing different content to different web browsers.
Show Web Inspector: Opens the Web Inspector. The Web Inspector lists the categories of resources found on a webpage, such as documents, style sheets, and scripts. It lets you view and search the page’s source code, Cascading Style Sheet (CSS) information, DOM trees, visual DOM metrics, and DOM properties. The Web Inspector also contains the error console and network timeline.
Show Error Console: Opens the Web Inspector’s display of HTML and XML syntax errors and warnings. The error console also displays JavaScript errors output from console.log, console.error, and console.warn.
Show Network Timeline: Opens the Web Inspector’s timeline of when your page’s subresources were loaded. This is useful for investigating how to improve the speed at which your webpage loads.
Show Snippet Editor: Opens a window you can use to quickly test small fragments of HTML, without requiring you to open an entire webpage.
Disable Caches: Causes Safari to retrieve a subresource from the web server each time the subresource is accessed, rather than using a cached copy.
Disable Images: Causes Safari to show the alternate content where images would be. This is useful for making sure your webpage has appropriate alternate content.
Disable Styles: Causes Safari to ignore all Cascading Style Sheet (CSS) styles. This is useful for investigating some types of page layout problems on your website. If you have a style sheet set in Safari advanced preferences, it continues to be used.
Disable JavaScript: Causes Safari to ignore all JavaScript. This is useful for investigating certain problems with how parts of websites behave, and for testing how a website performs on web browsers that don’t support JavaScript or have JavaScript disabled. You can also turn JavaScript off and on in the Safari security preferences.
Disable Runaway JavaScript Timer: The Runaway JavaScript Timer is used to interrupt the execution of very slow JavaScripts, so you can regain control of Safari. Disabling it is useful for some types of automated testing.
Disable Site Specific Hacks: Some versions of Safari contain special-case code that allows certain webpages to behave normally while Apple engineers work with you to find a better long-term solution. This option turns off that special-case code, so you can test your long-term solutions.