Hi soumaya,
it is not possible set the content of a LayoutPanelItems inside the HTML tag of the element on initialization. The proper way to set the content is by defining it inside the json or dynamically change it later using the update method documented in the API or use the id of the target item and change it’s innerHTML property to the new content. Here’s an example:
1) Using the update method

const targetLayoutPanel = document.getElementById('#item1');
document.querySelector('smart-docking-layout').update(targetLayoutPanel, { items: [{index: 0, content: 'New Content Here ...' }]});

2) Changing the content directly using innerHTML

const targetLayoutPanelItem = document.getElementById('#item1');
targetLayoutPanelItem.innerHTML = 'New Content Here ...';

Best Regards,
Smart HTML Elements Team