It is required to specify the render format in Js Viewer in the following scenarios:

• Rounding issues in HTML output. It is not always possible to fix this in HTML, especially in Galley mode.
• The output is not WYSIWYG sometimes.

The Js Viewer supports three render formats - 'auto' (default), 'html', and 'svg'.  For a Page or an RDLX report, set the value to 'html'. For a Section report, set the value to 'svg'

To switch the mode, you should specify the renderFormat setting through the JS Viewer API: 

viewer = GrapeCity.ActiveReports.JSViewer.create({
    element: '#viewer-host',
    renderFormat: 'svg'
});