WebView content top-left alignment issue macOS 10.9

Web page contents are at centre if we open web page in WKWebView on macOS 10.10 and above or in web browser. But if we open in WebView on macOS 10.9, contents are getting aligned to top left instead of centre. Any things am I missing?

NSURL *url = [[NSBundle mainBundle] URLForResource:@"COMPUTER & MOBILE PANEL" withExtension:@"html"];
NSURLRequest *request = [[NSURLRequest alloc] initWithURL:url];

WebView *aWebView = [[WebView alloc] initWithFrame:webViewContainer.bounds];
[aWebView setFrameLoadDelegate:self];
[webViewContainer addSubview:aWebView];
[[aWebView mainFrame] loadRequest:request];

Sample code here... https://drive.google.com/file/d/1QZkPy9dusBZbG274dOsru9gmK8pYZII-/view?usp=sharing