|| BOM (Browser Object Model)
	The BOM (Browser Object Model) consists of the objects navigator, history, screen, location
		navigator	-> The navigator object contains information about the browser.
		history		-> The history object contains the URLs 
		screen		-> The screen object contains information about the visitor's screen.
		location	-> The location object contains information about the current URL.
|| Dom 
	what is dom?
	The DOM is an API (application programming interface). 
	যেটি  একটি ডকুমেন্টের কাঠামো, ধরণ ও কনটেন্টকে পরিবর্তন করে।
	when is creates?
	When a web page is loaded, the browser creates a Document Object Model of the page.
	how you can use the document object ?
		-> Finding HTML Elements
		-> Changing HTML Elements
			element.innerHTML =  new html content
		-> Adding and Deleting Elements

How to display best selling products in magento

1. Create a new file for Block:

 * Catalog Product Bestseller Block
 * @author Amasty Team
class Mage_Catalog_Block_Product_Bestseller extends Mage_Catalog_Block_Product_Abstract
    public function getCollection()
        $storeId = Mage::app()->getStore()->getId();
        $collection = Mage::getResourceModel('reports/product_collection')
            ->setOrder('ordered_qty', 'desc');
        if (Mage::helper('catalog/product_flat')->isEnabled()) {
                ->joinInner(array('e2' => 'catalog_product_flat_' . $storeId), 'e2.entity_id = e.entity_id');
        } else {
                ->addAttributeToSelect(array('name', 'price', 'small_image'));
        if ($categoryId = $this->getCategory()) {
            $category = Mage::getModel('catalog/category')->load($categoryId);
        $collection->setPage(1, $this->getLimit());
        return $collection;

2. Create a template file 

 * Bestseller Products block template
 * @see Mage_Catalog_Block_Product_Bestseller
<div class="block block-list block-viewed">
    <div class="block-title">
        <strong><span><?php echo $this->__($this->getHeader()) ?></span></strong>
    <div class="block-content">
        <?php $products = $this->getCollection(); ?>
        <?php if (0 < $products->getSize()) { ?>
            <table class="bestseller-table">
                    <?php foreach ($products as $p) { ?>
                        <td style="padding: 15px 15px 0px 15px;">
                            <a href="<?php echo $p->getProductUrl() ?>" title="<?php echo $this->htmlEscape($p->getName()) ?>" class="product-image">
                                <img src="<?php echo $this->helper('catalog/image')->init($p, 'small_image')->resize(125) ?>" width="125" height="125" alt="<?php echo $this->htmlEscape($p->getName()) ?>" />
                            <h3 class="product-name">
                                <a href="<?php echo $p->getProductUrl() ?>" title="<?php echo $this->htmlEscape($p->getName())?>">
                                    <?php echo $this->htmlEscape($p->getName()) ?>
                            <?php echo $this->getPriceHtml($p, true) ?>
                    <?php } ?>
                    <?php foreach ($products as $p) { ?>
                    <td style="padding: 0px 15px 15px;">
                        <?php if($p->isSaleable()): ?>
                            <button type="button" title="<?php echo $this->__('Add to Cart') ?>" class="button btn-cart" onclick="setLocation('<?php echo $this->getAddToCartUrl($p) ?>')">
                                <span><span><?php echo $this->__('Add to Cart') ?></span></span>
                        <?php else: ?>
                            <p class="availability out-of-stock"><span><?php echo $this->__('Out of stock') ?></span></p>
                        <?php endif; ?>
                    <?php } ?>
        <?php } ?>

3. Now you need to call the created block, to do that go to Backend > CMS > Pages > Edit Page ‘Home page’ and add the following lines of code:

{{block type="catalog/product_bestseller" template="catalog/product/bestseller.phtml" header="Bestsellers" limit=4}}

4. You need to create a permission for this block:
   Backend – > System – > Permissions – > Blocks – > 'Add New Block' button