It’s an LGTB. Since the abbreviation LGTB (Lesbian, Gay, Bisexual, and Transgender) begins with a vowel sound (we say "an L"), it takes the article "an".
It’s an LDL. Because the initial letter of the abbreviation LDL (High-Density Lipoprotein) has a vowel sound (an L), we use the indefinite article "an".
It’s an HTML. Since the abbreviation HTML is pronounced as individual letters, and the "H" starts with a vowel sound ("eich"), we use the indefinite article "an".