Please suggest me a freeware tool for testing html and web contents on different platforms?

Hello,

I'm assuming by when you write different platforms you mean different screen sizes, such as mobile, tablet and desktop?

If so, and you are using chrome. You can open developers tools by right clicking on the white space of your webpage and click 'select element'.
Then click the phone icon, on the left side of the tabbed panel.
You can then select your device or resolution and be able to test your html code for the different resolutions.