Magento 2.1 – Override Knockout HTML Template in Theme

You can try this :

<!-- ko if: falg -->
...
...
<!-- /ko --> 
<!-- ko ifnot: falg -->
...
...
<!-- /ko --> 

In js file you can use like this :

 return Component.extend({.....}),
    initObservable: function () {
        var me = this;
        me.flag(true);
        return this;
    }, ....

After this you need to deploy static content or to test you can add changes directly in the same js and html file of pub/static.

Looks as though inside of your theme default.xml you are calling header.container again.

This will result in the error, you're trying to overwrite a reference block that already exsists - doubling up on the page the knockout that is being called, so what you actually need to do is the following:

add a new folder inside of your magento_theme called page_layout and copy the files from the core into there:

1column.xml

2columns-left.xml

Then you need to change the container for the header to be called something different, for example:

header.container.new

inside of your default.xml like so:

Then you need to add inside of your 1column change:

<container name="header.container" as="header_container" label="Page Header Container" htmlTag="header" htmlClass="page-header" before="main.content"/>

to

<container name="header.container.new" as="header_container_new" label="Page Header Container" htmlTag="header" htmlClass="page-header" before="page.top"/>

This will result in the error going and the header now working correctly.